Wouldn't the number be smaller if you consider that there are only 12 notes? As a musician I often notice several songs will have the same cord progression and melody. It comes up a lot. For instance people will here chicago's song "25 or six 2 four" and green days "Brain Stew/Jaded". It happens so often that Ive taking to saying that everything has been done since there are only 12 notes.

It often sparks debate about if the song was stolen. Whole websites like www.thatsongsoundslike.com are dedicated to this phenomenon.