Generally there is a pitcher rotation for each Baseball team. The day before the game the team officially announces their starting pitcher. For any number of reasons the team may decide to change the pitcher. This can happen until the start of the game.
We do not keep statistics on how often there are pitching changes. You can track this on our site over a few days, if you are interested. Pitching changes are noted with the new pitcher's name in all CAPS.
Since some clients only want a bet to have action if the announced pitcher(s) goes, and others want action regardless of who pitches, there are specific options a client may use when placing Baseball bets. Based on the option used to place the bet, there are also specific rules regarding how bets are graded when there is a pitching change.
For more information on Major League Baseball you might try their official site, www.mlb.com or use an internet search engine.