What Saturn 5 rocket is at the Kennedy space station ? Is it one that was used at any specific Apollo mission?

The first stage was an engineering test article that was test fired but never intended for flight. The second and third stages were intended for a later Apollo mission which never happened. The lunar module on display was the last H model intended to fly on Apollo 15, but it stayed on the ground after the final Apollo missions were cancelled and Apollo 15 became the first J mission. Under the front of the Saturn V is CSM-119, an unflown command and service module that would have been used with the SA-209 Saturn I rocket (on display in the rocket garden) had Skylab needed a rescue mission. Apollo 14's flown command module is also on display at the exit to the lunar theater off to the side.

The first stage was used in a static test fire. None of the displayed Saturn hardware actually flew. All used rockets lie somewhere on the bottom of the Atlantic Ocean. There is a used Apollo command module on display from Apollo 14.

Apollo 14 - the third manned mission ,during which two astronauts landed on the surface of the Moon

The Saturn 5 rocket at KSC is the actual rocket that would have been used for the cancelled Apollo 19 mission. (The Apollo 18 rocket was used to launch the Skylab mission).