Pitch count
From Academic Kids

A pitch count is a baseball statistic, most commonly applied to starting pitchers, and is simply a count of how many pitches the pitcher has thrown in a given game.
Except under unusual circumstances, a minimum of three pitches are needed to get three outs. It is very common for a pitcher to need 20 pitches to get through an inning.
Pitch counts are a concern for young pitchers, pitchers recovering from injury, or pitchers who have a history of injuries. Often a starting pitcher will be removed from the game after 100 pitches, regardless of the actual number of innings pitched. Pitch counts are generally less of a concern for veteran pitchers, who after years of conditioning are often able to pitch deeper into games, but this is not universal. Pedro Martinez is a famous exception, and is often lifted after throwing 100 pitches. A pitcher's size, stature, and athleticism can also play a role in how many pitches a pitcher can throw in a single game while maintaining effectiveness and without risk of injury.
Pitch count can also be used to gauge the effectiveness of a pitcher. It is better under most circumstances for a pitcher to use the smallest number of pitches possible to get three outs.
Opposing teams also pay attention to pitch counts, and may try to foul off as many pitches as possible (or at least any difficulttohit pitches) either on the theory that a tired pitcher is less effective, or to inflate the pitch count and drive a pitcher from the game in favor of a more rested, but possibly less effective relief pitcher.
The concept of a pitch count originated with Steve Busby, a promising young pitcher for the Kansas City Royals who won 56 games in his first three full seasons but suffered from a rotator cuff tear at age 27 due to overwork. His doctor suggested counting the number of pitches Busby threw as a means of gauging his recovery. Prior to his injury, Busby is known to have thrown 200 or more pitches in a gamedouble the number of pitches recommended today.