The question should be how many carriers fly from the U.S. to FRA. The answer would be any U.S. or German carrier that wants to. The same holds true to CDG,AMS,MAD,MUC, etc.
It does not hold true to LHR where only 4 carriers are allowed.
The following carriers fly to FRA from the U.S. ie Delta, Northwest, American, United, Continental, U.S. Airways, Lufthansa, Condor, Fedex, ATA, World.