I just did some research and it would appear there are 6 747s that can all serve as Air Force One (a designation reserved for any U.S.A.F. aircraft the U.S. president flies on).

There are two types of these 747s. The 2 VC-25A are the basic executive transport with the familiar blue & white colours. According to the U.S.A.F., there are 4 E-4Bs. These are the birds that are designed to be an airborne command post in event of a nukular-war-with-the-Russkies or other such emergencies. 2 were delivered in 1974 and were E-4As (later upgraded to -B) and 2 further 747-200s which were delivered in 1990. When I was on a Boeing tour in Renton a long time back, a guide told me the a/c are basically delivered as shells and the military finishes (or supervises) installation of everything including the avionics and flight controls. I don't know if this is true or not.

I've also heard that when the executive travels outside the U.S., there's a 2nd 747 that carries the press corp and entourage overflow from the AF1, and a C-5B that carries equipment such as armoured limos and all that good stuff.

There are also assorted a/c based on 757-200s and 737NGs that are used for executive transport.