There are 10^10^16 of them (but #1,000,443,163,313,125,343,132 is the evil one)
For some time, physicists have theorized about the existence of alternate universes. In fact, some models of physics require multiple universes, to explain some rarely observed phenomena. But, other than obvious ones like The Man In The High Castle Universe where the Nazis won WWII, the Earth-295 Age of Apocalypse Universe, and the Terran Empire "Mirror Mirror" Universe, just how many alternate universes are there? Well, some Stanford University physicists have answered that question, and the magic number is: 10^10^16 other realities.