How many pilots flew missions in the X-15?

From 1959-1968, twelve pilots flew three X-15 planes, flying record high-altitude flights, high-speed flights, and sub-orbital spaceflights. Collectively, pilots and craft performed a total of 199 free flights after being carried aloft and then air-launched from one of two modified B-52 mother ships. The pilots and craft also performed twelve scheduled captive carry test flights and 125 aborted flights (frequently due either to technical problems or poor weather) in which the X-15 did not uncouple from its B-52 mother ship, for a grand total of 336 flights. The X-15 program's flights generated data and flight experience which supported future development of aircraft, spacecraft, and human spaceflight.