From BR Bullpen
The pitch count is the number of pitches an individual pitcher throws in a game. It does not include warm-up pitches or pick-off attempts, only pitches thrown in anger, as it were. While pitch counts have received increased attention in the 1990s and 2000s, the idea that limiting the number of pitches thrown by a young pitcher is a good thing goes much further back.
In the 1960s, it was a policy of the Kansas City Athletics system under Eddie Robinson to limit young pitchers to 100 pitches, drawing widespread criticism when George Lauzerique was yanked in the middle of a no-hitter. The idea spread, with 100 being considered the arbitrary cut-off point - managers who let a young pitcher throw over 100 pitches were deemed cruel by many, while those who yanked one right before 100 were wise in saving their young pitcher's arms. The use of such an arbitrary number has not been justified statistically, even though it was the basis of the Pitcher Abuse Points system designed by Baseball Prospectus. It is common practice in baseball now to limit the pitch count of young prospects, of players coming back from injury or in spring training. Even seasoned veterans with no history of arm problem rarely throw more than 115 pitches in a game anymore, while pitch counts of 150 or more were common until the early 1980s.
Many organizations have limited pitch counts for young prospects in the minor leagues or for older pitchers coming off of surgery or injury rehabilitation. The Cuban Serie Nacional set a pitch count limit of 100 which was later increased to 120 in 2008. Some international tournaments have also limited pitch counts.