More than 20,000 new apartments in New York, both for sale and for rent, will open their doors this year — and likely just a fraction will find residents by year end.

The affordable ones, of course, will attract thousands of applicants, but the bulk of the new units — in gleaming towers, many of them conceived at the height of the last housing boom — will enter a saturated market, where buyers and renters have ample choice and little incentive to rush.

At the current rate of sales, it will take more than six years to sell all of the new development in Manhattan alone, which totals almost 8,000 units, said Jonathan J. Miller, the president of Miller Samuel Real Estate Appraisers and Consultants.

The pace of sales has already slowed coming into the new year, with homes seeking $4 million or more taking an average of 447 days to go into contract in 2018, according to Olshan Realty. In the halcyon days of 2013, similar homes spent only 172 days on the market.