Short answer: there were six flyable space shuttles built by NASA, though five actually flew in space missions. The sixth, Enterprise, was used for atmospheric approach and landing tests and never went to space.

Details:

• Operational fleet: Columbia, Challenger, Discovery, Atlantis, and Endeavour. These five completed spaceflight missions between 1981 and 2011.

• Prototypes and testing: Enterprise was built as a test orbiter but never flew in space; it served for atmospheric flight tests before being retired.

• Notable losses: Challenger and Columbia were lost in 1986 and 2003 respectively, with a combined 14 astronauts perished.

• Total missions: NASA’s Space Shuttle program flew 135 missions from 1981 to 2011.

If you’d like, I can pull a concise matchup of each shuttle’s service years, notable flights, and fate in a quick table.