Sales of apartments slowed in Manhattan in the first quarter, but just how much of a slowdown is in the works is up for debate. Tighter lending and falling consumer confidence have had an impact. “I don’t know anyone in the industry who can say that their business was the same as it was last year,” said Doug Heddings, a senior vice president at Prudential Douglas Elliman. But brokers say it could be six months or a year before the real effect of layoffs in the financial sector is seen. Analyst Jonathan Miller, who prepared Elliman’s market report, said the city’s supply of homes “seems on the high side.” That supply could increase, as 10,200 condos are expected to be built, more than double the 4,100 built in 2006, according to REIS Inc., a real-estate research firm.