Multiple Americas are the different versions of the Keystone World's America. There are an infinite number of them and they represent different levels of the Dark Tower. They usually differ from each other in small respects, like having different people on the currency, slightly different histories, different Presidents - for instance, Father Callahan once passed through a version of America where a man named Chadbourne had once been President. Chadbourne apparently so distinguished himself that he wound up on the ten dollar bill; however, in the reality in which Callahan originated, there had never been a President Chadbourne at all.
It is possible to travel between the Multiple Americas on the Highways in Hiding.