The FCC has a hand in how many frequencies in a given market you can control. I'm not sure what they say about broadcasting the same content over many frequencies in different locations, but Clear Chanel is basicaly a proof-of-concept that you can do it. (ok, not compleetly, but close).

Here in the bay area we have one radio station who's name I can't remember at the moment who's bought up nearly the entire 92.* space, and you can listen to them through out a range of 200 miles. At one point they had a transition that joked that the only reason they didn't own more than X number of frequencies was that the FCC wouldn't let them.

Matthew