After years of guesswork and speculation about the size and needs of Washington's Latino population, the city yesterday officially commissioned a survey of the two principal Hispanic neighborhoods to determine the facts.

But the facts themselves, some Latinos fear, may not be altogether welcome. The survey results could either greatly bolster or severely reduce Hispanic claims to a share of social services, housing money and political power in the city.

"This," said Alberto Gomez of the Council of Hispanic Community and Agencies, "is going to be our trial by fire," proving or destroying assumptions about the community that have become commonplace mainly through repetition and personal observation rather than hard evidence.

One rule of thumb used by the city's Latino leadership in the past has placed the Hispanic population at 10 percent of the city's total, or about 75,000 people, with the largest concentration in the Mount Pleasant and Adams-Morgan neighborhoods.

Carlos Rosario, acting director of the District's Office of Latino Affairs, which commissioned the $75,000 study from Development Associates Inc. of Arlington, recently guessed that the total Latino population is between 50,000 and 60,000 and growing.

But other Latinos fear that widespread displacement resulting from housing speculation may have forced the number of mostly low-income Spanish-speaking residents in the Adams-Morgan area to decline in recent years. If so, then the survey results could reduce the strenth of the community when dealing with government agencies.

The contract signed yesterday in Rosario's office calls for the survey to be completed within six months, after sampling some 4,000 households, with funding supplied by the city's Department of Housing and Community Development.