Ever since the last population count, when the Census Bureau reported 9.1 million Hispanic Americans in the United States, there has been speculation that 10 million to 15 million Hispanic-origin residents were missed.

At time passed, some publications and official groups began using higher estimates of the Hispanic population -- ranging to 19 million by Time magazine a year ago and a 20 million estimate by a State Department official last month.

Newsweek and other publications predicted that Hispanics would surpass blacks (25 million) to become the largest minority group in this country.

The assertions of an emerging Hispanic population gained such currency that last week U.S. Cenus Director Vincent Barabba felt it necessary to point out that, disregarding illegal aliens, it would be an estimated 78 years at current birth rates before Hispanics catch up with blacks.

The Hispanic population in the United States probably is between 14 million and 16 million, according to the nation's best census and demographic experts. This estimate includes all Hispanics in the 50 states.

In 1970, the census reported 9.1 million with an undercount that Barabba said last week probably was from 1.9 percent to 7.7 percent.

Every year the Census Bureau conducts a population survey. The most recent, 1978, shows 12.1 million Hispanic-origin persons. But there may be 2 million to 4 million more illegal ("undocumented workers") U.S. residents, putting the total between 14 million and 16 million.

Hispanic origin means persons of Mexican, Puerto Rican, Spanish and Latin American origin.

As the United States prepares for the next decennial census, the dispute over Hispanic poplulation threatens to erupt again.

The most accurate count or the best estimate is of more than academic interest to the country.

According to Barbabba, population figures help determine seats on local, city and county government bodies, in state legislatures and in Congress.

An undercount could mean disproportionately low representation.

It also could affect funds for bilingual education, federal revenue sharing and aid to education, equal opportunity, Spanish language balloting and a host of other programs, according to Ed Fernandez, chief of the Census Bureau's ethnic and Spanish statistics branch.

Al Perez of the Mexican-American Legal Defense and Educational Fund, which has been working on the Census Bureau to get a precise count in 1980, agrees that the figure of 20 million to 25 million Hispanic-origin residents is exaggerated.

He said studies do show that net migration from Mexico is far less than total migration and one study concludes that the average undocumented workers stays only four months.

Putting all things together, he guessed that total Hispanic population here would be around 16 million.

Experts both in and out of the Census Bureau guess the maximum figure is less, perhaps 14 million to 15 million at the outside.

The Census Bureau, however, would like to be sure, and it is making an extra effort to identify all persons of Hispanic origin.

Fernandez said the bureau is undertaking an immense "outreach" effort through Hispanic organizations and media, to convince everyone regardless of legal status that they should answer the questionnaires (to be sent to about 95 percent of the population by mail) or respond to the interviewers who will visit the remaining households.

Although people must give their names, Census Bureau information about individuals by law is confidential and cannot be given to any other agency.

In 1941, he said, the bureau refused to give the FBI information about Japanese-origin residents despite the Japanese attack on Pearl Harbor.

An example of the outreach program is the National Spanish Television Network, with 29 stations. Starting in February it will do 3,000 spots asking people to answer the census and to register and vote in the election.

To get a more accurate census overall, not just for Hispanics, the bureau is checking mailing lists, sending people to check supposedly "vacant" domiciles and will check driver's license records and other public documents to try to fill gaps.

Fernandez said the general census "short form" contains a question, No. 7, which reads: "Is this person of Spanish/Hispanic origin or descent?" It also asks for a breakdown of whether "Mexican, Mexican-American, Chicano" or "Puerto Rican" or "Cuban" or "Other Spanish/Hispanic."

About 20 percent of the households will get a "long form" which asks the No. 7 question but also asks whether the respondent is foreign-born and where, what language is spoken in the home and what the person's "ancestry" is, such as "Mexican, Nigerian, Polish, Ukrainian, Venezuelan," etc.

From this extra effort, the Census Bureau hopes to get a good count -- perhaps one that will end the dispute over the size of the Hispanic-American community.