The Apollo spaceflight program lasted from 1961 to 1972. Apollo 11 was Neil Armstrong's famous first step. Apollo 17 was the last manned mission to the moon.
In all, 20 Apollo flights were planned but Apollo 18, 19 and 20 were canceled.
But in 2007, a retired astronaut blew the whistle and claimed Apollo 18, 19 and 20 did happen. But they were highly classified.
He knows this because was the commander of Apollo 20.
He said the reason these missions were kept secret is because of what they found on the moon.