According to the FCC there are 17.777 FM-transmitters in U.S.A. 6.633 are Commercial radio, 4.085 Educational (mostly NPR and universities) and 873 Low-Power FM (Community radio). Also there are 6.186 FM translators and boosters.
Also there are 4.715 AM stations in the medium wave band.
Read more: Radio Survivor
Today there are 1.900 radio stations in the U.S. using the digital system HD Radio transmitting each 1-4 program channels on the same frequency channel in the FM band.