ONE DAY in March, John Williams, film composer and conductor of the Pops, left his baton in Boston and came to Washington to march arguments before Congress against the proposed 50 percent cuts in the budget of the National Endowment for the Arts.

Congress seems to bring out the statistics in everyone, and Williams was no exception. He began that portion of his testimony by quoting figures widely used by those in government, business and the press.

"Since the Arts Endowment was created, corporate support for the arts has risen from $22 million a year to $436 million."

These figures seem to support assertions by the Endowment and corporate benefactors alike that business gifts to the arts are booming -- as well as the Reagan administration's claim that the private sector will make up the cuts in the Endowment budget.

Williams continued: "The Arts Endowment has challenged the orchestras to increase private funding and find new sources of support. Orchestras have met the challenge. In 1965, the year before the Arts Endowment was created, corporate gifts to orchestras totaled $2.4 million. Today, that total is $15.6 million."

Williams intended to show that Endowment grants are a stimulus, not a substitute, for increased corporate funding.

But by combining the two sets of statistics, one emerges with a less rosy picture than Williams had intended: In 1965, symphonies won 10 cents of every dollar corporations gave to the arts. Today, for all the challenge of the Endowment, symphonies win only 3 1/2 cents.

What happened to make the symphonies apparently fall so low in the arts donation sweepstakes? The answer may lie in the validity of Williams' two sets of figures.

The statistics Williams used came from two different sources. One is the Business Committee for the Arts, which met here recently. Since 1967, it has commissioned a triennial survey of corporate support for the arts, based on information volunteered by corporations. Williams used the committee's findings that corporate support has zoomed from $22 million to $436 million.

But when it came to specific support of symphonies, the Boston Pops conductor cited figures from the American Symphony Orchestra League. This group charts the rise in corporate gifts to symphonies based on information from its member orchestras. The league provides the figure of $15.6 million for 1979.

The business committee, however, says symphonies got $52 million from corporations in 1979 -- more than three times the league's estimate.

As the country commences a major public policy debate on the role of government financing of the arts and humanities, it turns out that both sides are using figures that appear to conflict.

One of the closest students of statistics about the arts is Dick Netzer, dean of the Graduate School of Public Administration at New York University and author of "The Subsidized Muse," one of the few in-depth looks at government support for the arts.

Netzer is not satisfied with the information available on the economics and financing of the arts.

"Really," he says, "the only absolutely solid information we have on income of arts organizations comes out of a series of Ford Foundation studies. Those studies stopped in 1964."

Lisa Gonzales, research director at the symphony orchestra league, doesn't dispute that business gave $52 million to symphonies in 1979, even given the discrepancies with her own group's statistics.

"Our figures don't include money given to symphony endowments and capital fund-raising campaigns." Gonzales explains.

Would this make up the difference? During 1979, 26 symphonies won National Endowment for the Arts challenge grants, which require recipients to conduct intensive fund-raising drives for three years, and to match the sum of the grant three times over. Some of the money raised went to annual operating expenses, but most symphonies used the campaigns to build their endowments. According to the records of the National Endowment, the 20 largest symphonies which made up the bulk of the program at that time, were challenged with $12.7 million in federal money. They claim to have raised $82.5 million in matching funds over three years. Of that total, symphonies claimed $4.7 million was raised from corporations in 1979.

The $15.6 million the Symphony Orchestra League accounts for, plus the $4.7 million that may have gone to symphony endowments in 1979, still leaves unaccounted for over half the money the business committee estimates corporations gave to symphonies.

And to assume that much of that money went to endowment and capital campaigns of America's 1,552 other symphony orchestras doesn't work. Most of those orchestras don't boast endowments or symphony halls all their own. Most are not even considered professional by the National Endowment for the Arts.

Gary Fiebert of Touche Ross, the company that does the business committee's polls, says that at least part of the difference in statistics is that it is up to the member corporations themselves to determine what will be included in the arts-giving statistics.

In reference to another discipline, the business committee estimates that corporations gave $30 million to theaters in 1979.

"That doesn't tally or jive with what we know from our studies of nonprofit theater," explains David Visser of the Theater Communications Group, which anually surveys theater income. "I can't imagine that there are that many recipients of corporate grants that we don't know about."

Visser found that 154 nonprofit theaters (about 75 percent of the total) received $4 million in corporate contributions in 1979. Visser speculates that the business committee's figure of $30 million might include donations to the Kennedy Center and the Lincoln Center, which are not solely theater organizations, and the $10 million that Mobil now spends on Masterpiece Theater and other public television ventures.

The business committee poll does not pinpoint where corporate money goes. Touche Ross sent questionnaires to 17,172 corporations, 1,343 responded and enough nonrespondents were polled "to make the survey statistically valid." Responding corporations answered three basic questions: 'how much they gave in philanthropic contributions to the arts in 1979, how much in business expenses were spent for the arts and what percentage of all that money went to 14 categories ranging from opera to "others."

In answer to Visser's question as to whether the $30 million that went to theaters wound up in the coffers of the Kennedy and Lincoln centers and Masterpiece Theater, Fiebert says that that money would have been tabulated under the headings "cultural centers" or "public television." The business committee poll estimates that cultural centers received $44 million and public television received $48 million. Incidentally, in 1979, the Lincoln Center reported receiving $2.8 million and the Kennedy Center received $1.5 million.

Fiebert points to two major reasons for the discrepancies between the business committee's estimates and what groups such as the Symphony Orchestra League and the Theatre Communications Group tabulate.

"We measure philanthropic contributions of business, and business expense support. That includes all expenses a company can bookkeep, not just grant," Fiebert explains.

So while the Symphony Orchestra League tabulates only the $3 million that AT&T gives to orchestras on tour, the business committee includes the million dollars AA&T spends to publicize those gifts, according to Edward Block, vice president in charge of the company's corporate gifts.

According to Fiebert, the survey results could include the price of symphony or theater tickets a businessman might deduct as a business entertainment expense. The symphony can never know that it is the recipient of that kind of corporate largesse.

Second, Fiebert explains, "the theater groups and the symphony groups have a smaller data base than we do. They include what their members, the big-name theaters and synphonies, get. We include anything a business defines as a theater or a symphony, including amateur groups in small towns."

While the Theater Communications Group limits its study, the Symphony Orchestra League tries to estimate the operating cost of all symphonies.

The Business Committee for the Arts is not primarily in the business of publishing statistics. Its officials stated in the report of its 1980 activities that its prime mission continues to be "to communicate to U.S. business leaders that support of the arts of their own choosing in locations sensitive to their interests is needed and is good business."

The business committee has been at it since 1967. David Rockefeller and Douglas Dillon first organized the committee, and almost 100 corporations signed up. Today, there are 144 members. Their recent annual meeting ended with a glittering dinner in the East Wing of the National Gallery. Indeed, the committee is playing a more public role than ever before in the arts-support game, both in terms of the larger role the Reagan administration anticipates corporations will play and in terms of the varying views among committee members about the Endowments.

Officials of the National Endowment for the Arts claim they have forged a partnership between business and government in support of the arts. They meet with the Business Committee for the Arts every three months.

Livingston Biddle, chairman of the Endowment, attends the meetings and says he finds them helpful. Later in the conversation he observes, "We've never established a real structure for business cooperation with the Endowment."

Asked about the Business Committee's statistics, Biddle says, "They seem right to me . . . . When the Endowment started, all this support for the arts wasn't there. I think the government's support is responsible for all the increase. Working by a sort of osmosis, [corporate] support has increased in geometric proportions."

While the two organizations do occasionally work together, business commitee President Ed Strauss takes no position on the cuts in the Endowment. "Some of our members support the Endowment and use its expertise and other companies don't," he said. As an executive for ALCOA, Strauss spent nine years in Europe, a period which created doubts in his mind about the wisdom of government support for the arts: "I didn't hear one symphony over there that was as good as our top four symphonies. There's no volunteerism in Europe. The arts rely too much on the government."

Some in business, like Herb Schmertz, architect of Mobil's arts-support program, hope the Endowment "is phased out in a couple of years.

"Government hasn't had anything to do with increasing corporate support for the arts," says Schmertz. "A good portion of government money is not going to arts but to the arts bureaucracy; there's a whole herd of people on the government side evaluating these increasingly complex grant proposals. And then on the other side there are people in the arts whose only job is writing grant proposals. Government shouldn't be telling people in the arts what to do."

As to Mobil's plans for future arts subsidies, Schmertz will only say that they have been increasing every year and that the company will decide it budget for 1982 at the end of this year.

By law, at tax time, corporations can deduct up to 5 percent of their pretax profits in gifts to charitable organizations such as the arts. But, according to one trade organization, the American Association of Fund Raising Counsel, the average level of corporate contributions has been one percent of pretax profits over the 1970s. That is slightly less than the peak in recent years, 1.26 percent in 1969, according to Internal Revenue Service statistics. Only about a tenth of the average one percent in charitable contributions goes to the arts -- a portion that has been increasing over the years from around a 20th in 1970.

Schmertz says Mobil is aware of the 5 percent in allowable deductions but that it is not a "magic goal." The business committee's Strauss agrees. His group is gearing up for a new advertising campaign. The rallying cry will not be 5 percent. It well be: "Take a hard look at the arts and see if they can't help your business pragmatically and philosophically."