NASA had two Shuttle Carrier Aircraft. One was a modified Boeing 747-123. The other a 747-100SR-46. The SCAs were used to ferry space shuttle orbiters from landing sites back to the launch complex at the Kennedy Space Center and also to and from other locations too distant for the orbiters to be delivered by ground transportation.
NASA 905, a Boeing 747-123 model has been retired. It is on display at Space Center Houston.
NASA 911 was the second SCA, a Boeing 747-100SR-46 version. It was retired and is on public display at the Joe Davies Heritage Airpark in Palmdale, California.
In all, NASA has been in possession of three Boeing 747s. The third was not an SCA, but an airborne observatory known as the Airborne observatory, Stratospheric Observatory for Infrared Astronomy (SOFIA). The aircraft is a Boeing 747SP-21 which is housed at the Armstrong Flight Research Center.
Currently, NASA lacks an active aircraft capable of carrying a Space Shuttle.