– Xochielt Sanchez
Answer by Robert Frost:
Not once, but six times. Apollo 11, 12, 14, 15, 16, and 17 all landed on the lunar surface. Apollo 13 should have, but had technical difficulties. So, that would have made it seven times.
As to why Apollo 18 and subsequent didn't go to the moon…
The short answer for why Apollo 18, 19, and 20 were cancelled is money. As money dwindled, the farthest out missions were cancelled. Apollo 20 was cancelled and then, later, Apollo 15 and 19 were cancelled (16 became 15).
But, to be more specific. NASA's plans for the 1970s were to build a space station and space shuttle to serve it. The first step of that was Skylab. Apollo 20 was cancelled on 4 January, 1970, so that the Saturn V that was slated for that flight could be repurposed to launch Skylab.
There are other contributing reasons:
Lack of Support
The 1960s were a big thrust forward to accomplish the goal of "putting a man on the moon and returning him safely to Earth, before this decade is out." There was great public and political support for that goal. But once it was done, the public and their politicians quickly lost interest. They couldn't tell the difference between one Apollo mission and the next.
Attempts to Contract Federal Spending
The Nixon administration was burning money in the military conflict in Vietnam. In a story that should sound familiar, when you're burning money on a foreign war, you try to compensate by cutting domestic projects.
Apollo wasn't Nixon's baby. It was a project started by Democratic presidents. He wanted his own vision for NASA. That vision ended up being the Space Shuttle. Apollo was cancelled to free up resources for the Space Shuttle much like the Space Shuttle would later be cancelled to free up resources for the Constellation program (which itself would be cancelled).