To the great mysteries, enigmas and half-buried secrets of New York City add this: What is the real number of vacant apartments in the New York City Housing Authority's buildings?

Despite a waiting list of over 300,000 mostly low-income applicants, NYCHA is sitting on about 7,500 vacant apartments in its projects, according to a report issued last week by the Association of Community Organizations for Reform Now. ACORN's numbers are based on calculations using the city's own reports: they subtracted the number of occupied apartments in the system--about 171,692--from the total number of apartments that NYCHA lists as habitable--179,033--and you get 7,341.

The authority's explanation for the empty apartments seems to contradict their own statistics. NYCHA spokesman Hilly Gross says 941 of the units are in the process of being rented. The rest--which he numbered at only 4,500 apartments--are either awaiting repair or reserved for disabled tenants, he says. Gross offered no explanation for the 2,000-odd vacancies left over after his calculations.

Yet Gross dismissed the notion that the authority was hoarding apartments. "It is so absurd. It is such an insane argument," he said. "If you leave [these] apartments vacant, you have lost $4 million [in rental income]."

ACORN, which has clashed with NYCHA for control of tenant associations in several Bronx and Queens projects, charges the apartments are being warehoused so that the city can achieve its long-standing goal of replacing welfare-recipient tenants with residents who earn $25,000 a year or more. In July, a federal judge temporarily halted the authority's plan to bring in working-class tenants, but the scheme is expected to be green-lighted later this year. ACORN and other low-income tenants groups are fighting to see that it never gets enacted.

"The basic issue is low-income housing and the city is no longer in the business of low-income housing," says ACORN's Helene O'Brien. "You can see how that area is going to be set aside for middle-income people."

The large projects with the highest vacancy rates are: the Albany Houses in Brooklyn (150 out of 1,220 units); the Astoria Houses in Queens (66 out of 1,102); Beach 41st Street in Far Rockaway (111 out of 712); Carey Gardens in Coney Island (48 out of 674); Brownsville in Brooklyn (81 out of 1319); Edenwald in the Bronx (116 out of 2,034); Gowanus in Brooklyn (124 out of 1134); Lincoln in Harlem (98 out of 1283); Mitchel in the Bronx (109 out of 1729) and Van Dyke in Brooklyn (126 out of 1,713).