Seven astronauts (the Mercury Seven) were selected to fly during the Mercury program. Yet Slayton, one of the Mercury Seven didn't fly during Mercury (though he did fly for Apollo-Soyuz). Why were there only six crewed Mercury flights? Did one of the Mercury flights get cancelled, or were there only supposed to be six flights and was one extra astronaut selected as backup?
It wasn’t originally intended for the astronauts to be matched 1-to-1 with the flights; four additional crewed Mercury-Redstone (suborbital) and three more Mercury-Atlas (orbital) flights were planned but canceled. The additional suborbital missions were canceled to put a man in orbit sooner, and the last orbital missions were canceled as NASA shifted focus to the upcoming Project Gemini.
Mercury astronaut Deke Slayton was grounded for a heart condition, so it was actually somewhat coincidental that the program ended up with six flights and six available astronauts. Without the time pressure coming from the USSR’s Vostok program and Kennedy’s lunar challenge, most of the Mercury astronauts would probably have flown twice during the program.