Yes, pinch runners do have to replace whoever they're pinch running for unless of course the pinch runners gets substituted for a defensive replacement in the next half of the inning.
You can have as many pinch runners as necessary. Of course, there are only 25 active players in the roster. In the American League, 10 of those players are already started in the game, leaving 15 players. Technically you could use 15 pinch runners if you really wanted to. Once a player is removed from the game, he cannot come back.
It happened once in 2004
Yale University was the first to acquire a mascot. The official mascot was a bulldog and this was established in 1889.
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Lenny Harris
Three foot, seven inch Eddie Gaedel made one pinch hitting appearance for the St. Louis Browns on August 19, 1951. He was walked on four pitches and left the game for a pinch runner.
Texas Rangers
In MLB, it is a balk.
A defensive substitution or carry over of a pinch hitter.
Lenny Harris. Harris, who played between 1988-2005, holds the MLB record for pinch hits with 212.
Allen Craig did this evening (7/21/12) pinch-hitting for the Cardinals in the bottom of the seventh against the Cubs. He hit two doubles in a 12-run inning.