Skylab was actually a converted second stage of a Saturn booster rocket originally intended for one of the Apollo missions that were cancelled.

After Apollo 17 there have not been any plans for any moon mission. It was the Skylab and the Shuttle program.

Apollo 18 was cancelled to free one Saturn V to launch Skylab and to cut costs

Skylab was launched on 14th May 1973 on top of a modified Saturn V rocket using surplus material from the Apollo Lunar program.

Skylab re-entered Earth's atmosphere and was destroyed on July 11th 1979.

There were 5 unmanned missions following the ill-fated Apollo 1 disaster. After that, there were 2 earth orbital missions (Apollo 7 & Apollo 9), and 9 lunar missions (Apollo 8, 10, 11, 12, 13, 14, 15, 16, and 17). There were 3 long duration earth orbital missions after Apollo 17 that used the Apollo hardware and spacecraft. Skylab 1 was the workshop itself where the astronauts lived for months at a time. Skylab 2 lasted 28 days, Skylab 3 lasted 56 days and Skylab 4 lasted 84 days in space.

The last mission to use Apollo spacecraft was the first joint mission with the Soviet Union. An American Apollo docked with a Soviet Soyuz spacecraft. The 2 vehicles stayed docked for 44 hours before parting ways. The last Apollo spacecraft splashed down on July 24, 1975.

So total there were 19 "Apollo" missions. 13 manned missions and 6 unmanned.