AF1 is the call sign of whatever USAF plane the President happens to be flying on. AF1 has been a C-17 when Pres. Clinton used it to go into FRY.
There are two B-747s used by the Presidential Flight, both -200s (last production ones, I believe) tail nrs. 28000 and 29000. Usually, both are deployed whenever the President travels as a back-ups.
GF