An Apollo mission typically had a crew of three astronauts. The standard arrangement was a commander, a command module pilot, and a lunar module pilot.
The commander led the mission and flew the spacecraft alongside the command module pilot. During lunar missions, the lunar module pilot operated the lunar module, although the title was retained on missions that did not carry one. On Apollo 7, for example, Walter Cunningham held that title even though the mission stayed in Earth orbit and carried no lunar module.
The three-person structure allowed two astronauts to descend to the lunar surface while one remained in lunar orbit aboard the command module. On Apollo 11, Neil Armstrong and Buzz Aldrin landed in the lunar module, while Michael Collins stayed in orbit in the command module Columbia.
The crew size can be confused with the number of astronauts who walked on the Moon. Only two members of each lunar-landing crew reached the surface; the third astronaut remained in orbit. Across the nine crewed lunar missions, every Apollo crew consisted of three people.