More than 18 million Americans will be homeless or on the edge of homelessness within 16 years because of an increasing number of poor people and a decreasing number of affordable housing units, according to a national study released yesterday.

Neighborhood Reinvestment Corp., a nonprofit group funded by Congress, announced its findings on a street corner in Adams-Morgan, where aging buildings are being converted to pricey condominium complexes.

"We chose this site to emphasize the erosion of low-income housing," said William A. Whiteside, executive director of the group. "Just two years ago low-income people lived here."

Barry Zigas, director of the National Low-Income Housing Coalition, another major group monitoring affordable housing trends, said the new study may be conservative in its estimates, but "it supports what we've been saying all along. We are heading into a major crisis."

The number of poor households is expected to jump by 44.5 percent, from 11.9 million to 17.2 million, from 1983 to 2003, according to Phillip L. Clay, a Massachusetts Institute of Technology professor and author of the report. For a family of four to be considered poor at present standards it must earn less than $12,000, Clay said.

During the same 20-year period, the study said the number of low-rent units, currently those costing $325 or less a month, is expected to shrink from 12.9 million to 9.4 million.

To accommodate the rising number of poor people and the falling amount of low-cost housing, the study says, 7.8 million residential units will be needed to house 18.7 million Americans by 2003.

Without significant changes in federal housing policy and an unexpected surge in low-cost housing, Whiteside said these 18.7 million people -- the equivalent of the combined populations of New York City, Los Angeles, Chicago, San Francisco, Dallas, the District of Columbia, Philadelphia and Atlanta -- will be displaced, burdened with excessive rents or forced to live in slum conditions.

In Northern Virginia, as many as 10,000 low-income residents are now worrying about where they will move because developers are converting Arlington's Lee Gardens and Alexandria's Arlandria low-rent apartments into plusher residences, many costing $750 to $850 a month.

Expiring federal housing contracts and 1986 tax revision laws were cited in the study as major contributors to the demise of affordable housing.

As many as 900,000 of the 1.9 million privately owned but federally assisted low-income housing units across the country could be refinanced by 1995 because their government contracts will have expired. The owners would then be under no obligation to rent to poor tenants.

Likewise, tax advantages that for years lured investors to multifamily housing were killed last year, a cut expected to bring a sharp decline in privately owned low-cost housing.

The study, called "At Risk of Loss: The Endangered Future of Low-Income Rental Housing Resources," recommends several measures to minimize the loss of affordable housing, including:More federal and state incentives for nonprofit and private housing construction. State and local government purchases of low-cost housing. Federally subsidized refinancing for privately owned apartments about to be converted to higher-priced units because of expiring government contracts. A change in Department of Housing and Urban Development policy to tie rent subsidies to projects instead of individuals.

HUD has focused its recent efforts on Section 8 certificates and vouchers, rent subsidies given directly to tenants and applied to whatever apartment the tenant rents. The report argues that in addition to keeping low-income housing stock on the market, project-based aid also sends a message that there is a national policy to save low-cost housing.

Rep. Henry Gonzalez (D-Tex.), chairman of the House subcommittee on housing, agreed that many bold and expensive steps must be taken to solve the looming housing crisis. But he said the urgency is even greater than the report suggests because it uses a 1983 data base.

"The thing that impresses me," the study's director Whiteside said, "is that it scares the hell out of you to think about 18.7 million people, even if it's conservative."