According to a popular space exploration web site, 355 individuals few in a total of 833 crew slots in the Space Shuttle program. That's an average of about 2.5 flights per astronaut, though many only flew once; apparently seven times is the most anyone flew the shuttle (here and here).
As I understand it, these people trained for a mission for years. It seems like it would be more efficient to re-use crew members (perhaps not mission specialists, but pilots and commanders?) rather than always having to retrain people for the job. Why did NASA apparently prefer to train new personnel rather than to have more astronauts make several flights?