One of the most maddening -- and frightening -- aspects of the AIDS epidemic is that no one knows how many people have been infected with the deadly virus. It can lie dormant in the body for years before producing symptoms. U.S. health officials have estimated that between 950,000 and 1.45 million Americans have picked up the virus, but that is based on spotty data. Admits a federal AIDS expert: "It's just hard to take those numbers seriously."

But now there is a set of numbers that may deserve to be taken seriously. Joel Hay, a health economist for the Hoover Institution at Stanford, has used a statistical tool called "back calculation" to analyze data on AIDS infections. His surprising conclusion: about 640,000 Americans carry the virus. If he is right, the epidemic, while still devastating, may be only half as widespread as generally believed.

Hay's calculation is based on the observation that in a specific group of people carrying the AIDS virus, a certain percentage will develop symptoms within a given period of time. For example, it appears that roughly 6% will come down with the disease in the second year after infection. Since the Government has accurate figures on how many people have developed AIDS, Hay was able to work backward to figure out the number of those who carry the virus.

The economist uses his techniques to predict the rate at which new AIDS cases will be reported, and his projections seem to be on target. Says a Government scientist: "Every month Hay's numbers look better and better, while the official estimates look worse."

AIDS activists doubt Hay's figures and fear that they could cause the nation to become less concerned about the disease. The critics think that the use of anti-AIDS drugs has delayed the onset of symptoms in many people and thus made the economist's calculations erroneous.

The Government may settle the issue with a national AIDS survey, which began last week in Dallas. The residents of 3,400 households were sent letters asking them to take anonymous AIDS tests and answer questions about their sexual practices and drug use. Some AIDS groups protested, charging that the money used for the survey could be better spent on treatment and the search for a cure.