JoshBarbanel

It’s getting crowded at the top of Manhattan’s apartment market.

As condo developers chase billion-dollar paydays through the construction of luxury dwellings, the cranes dotting the city are sparking fears of a supply glut.

Builders are plowing ahead with scores of condominiums priced above $20 million in skinny glass towers throughout Manhattan.

One building, the 66-story tower at 220 Central Park South, is listing more than 60 apartments above $20 million, according to filings made with the New York attorney general’s office. By comparison, in 2008, just 29 new condos sold for $20 million or more across all of Manhattan, according to appraisal firm Miller Samuel.

“There are a finite number of people that will buy these,” said Jonathan Miller, president of Miller Samuel. As for the developers kicking off projects now, “You are going to have haves and have-nots.”

The boom in the upper echelons of residential real estate is touching a handful of cities around the world, including London, Singapore and Dubai, where the global elite are pouring money into top-quality homes. But the demand is particularly acute in New York, reshaping its skyline to an extent few would have predicted during the real-estate bust.

There are signs demand for this rarefied product might be nearing its limits. The 1,004-foot green glass tower One57 remained about 25% unsold for much of last year. Its builder, Extell Development Co. lowered its expected total revenue from the building by about 4%, or $100 million, in part because of the slowdown, according to filings made with the Tel Aviv Stock Exchange in Israel, where Extell has issued $300 million in debt.

Extell founder Gary Barnett said sales have picked up substantially in the past couple months. He declined to discuss specifics but said he hoped to fully sell out the tower by the end of the year.