620,000: That’s the number of restaurants in the United States, The New York Times reported this past week. In recent years, the number of chain dining spots in particular has grown twice as fast as the population, but not because Americans’ appetite is bottomless. Rather, big-time investors poured billions of dollars into the sector, and restaurant brands figured out how to expand rapidly while pushing the hassles and risks of operating individual locations upon their franchisees. As Ruby Tuesdays and Outback Steakhouses proliferated on Main Street, Wall Street got good returns, at least for a time. But the chain-restaurant bubble may be popping, as marketers realize that the average person can still eat only so many onion mums and buffalo wings.