On page 63,553 of yesterday's Federal Register--the last page of the last edition of the Register for 1981--there's a statistic that Ronald Reagan and the rest of his administration will be bragging about for years to come.

The text of page 63,553 contains only the tail end of a long, utterly routine explanation of records-keeping systems at the Commerce Department. The item that has the Reaganauts crowing is tucked away in the upper right corner: the page number itself. At 63,553 total pages, the Federal Register in 1981 was more than 23,000 pages shorter than it was in 1980, Jimmy Carter's last full year in office.

That 25 percent drop in the government's official compilation of new and amended regulations may or may not mean anything, depending on which expert you ask. But White House aides say the page count will be widely cited by administration officials as proof that the president has delivered on his promise to cut red tape and Washington paperwork.

"It's a testament to what we've done," exudes Danny Boggs, a senior policy adviser at the White House, in a typical commment. "When The Post runs those Federal Register page numbers every week in the Data Base on this page each Monday , that's like a paid advertisement for the way Ronald Reagan is cutting back on regulation."

Similar paeans of praise are becoming standard items in speeches that Reagan's people deliver around the country. Two "report cards" the administration put out on itself in recent days both cited the big drop in Register pages as prima facie evidence that--as the White House public affairs office put it--"regulatory activity has been cut substantially."

As in all other regulatory matters, that assertion is the subject of debate among students of the field. "To say a reduction in pages is per se beneficial doesn't begin to reflect the complexities of the situation," says Judith Areen, a Carter administration regulatory official now teaching administrative law at Georgetown University law school.

Areen points out that the Register includes much material that is non-regulatory. She notes, too, that a drop in page numbers may represent inaction rather than aggressive deregulatory activity; it can take as many Register pages to drop regulations as to add them. When the administration this summer published a major relaxation of regulations under the Davis-Bacon wage-setting statute, that "deregulatory" initiative used up 88 pages of one issue.

Washington consultant Fred Emery, a former editor-in-chief of the Register and a loyal daily reader of the compendium for many years, says the big drop in pages "may mean that they've turned the corner on the increase in regulation . . . or it may just mean a temporary slowdown because a lot of regs are still in the pipeline."

Emery and other Register buffs note a certain irony in the hoopla over the reduction in Register pages.

For the better part of a decade, government officials--including leaders of the last two Republican administrations--have praised the growth of the Register, in which government publishes its plans and programs for all to see, as one of the "sunshine" reforms aimed at opening bureaucratic decisions to public scrutiny and participation. Emery says the new decline in pages could mean "agencies are looking for ways to put out their paper . . . that look better statistically and don't end up in any official codification."

In any case, it appears the Register was thinner in every way this year. The publication is divided into categories: final and proposed rules; official notices (in which agencies announce such things as public meetings and grant awards); and presidential proclamations and orders. Based on figures through November, all were coming in shorter in 1981.

Even the Register's strangest category--pages and pages of nothing-- showed a decline. Almost daily, printing exigencies force the Register to assign dozens of page numbers to blank pages; thus the U.S. government, like some giant publisher of Zen textbooks, prints thousands of pages of official nothingness every year. In 1981, though, the blank page numbers totaled about 6,000--less than half the previous year's figure.