Ryan Serhant says he has a list of all the billionaires he’s met—all 101 of them. For the real-estate mogul whose brokerage, Serhant., is on track to close $6 billion in sales this year, building relationships with the ultra-wealthy is his “number one job.” Serhant, 41, transformed himself from a struggling actor who only earned about$9,000 in his first year of real estate—in 2008, the week Lehman Brothers collapsed—into one of America’s most successful brokers. He’s closed more than $15 billion in real e ...