MLB Roster Limits
Each MLB club is permitted to reserve no more than 40 players on its MLB Reserve List, which is why the MLB Reserve List is also called the "40-man roster."
Each MLB club is permitted to roster a maximum of 25 players on its MLB Active List (also known as the "25-man roster") Opening Day through August 31st, and the limit is increased to a maximum of 40 players September 1st through the last game of the MLB regular season. The Active List maximum limit then reverts back to 25 players again for post-season play (LDS, LCS, and World Series).
Beginning on MLB Opening Day up through August 31st, a club can temporarily add (recall or select) a 26th player to its MLB Active List on any day where two games are scheduled, as long as the second game was scheduled at least 48 hours in advance. The "26th man" must be on the club's MLB Reserve List (40-man roster) or has to be added to the club's 40-man roster that day. The "26th man" does not have to be a pitcher. The 26-man Active List limit is in effect for both games of the doubleheader, but the "26th man" cannot be switched between games. The club's MLB Active List limit goes back to 25 players the next day.
A player can be added to a club's Active List as the "26th man" even if he has not spent the minimum required 10 days on optional or outright assignment prior to being added. A player recalled as a "26th man" accrues one day of MLB Service Time.
A club can recall a player from a minor league Optional Assignment and place the player on its "Taxi Squad" for one day. If the player is not added to his club's MLB Active List by 3 PM EDT the next day or at least three hours prior to the scheduled start of the next day's game (whichever is later), the player must be removed from the Taxi Squad and returned to his minor league assignment. A player on the Taxi Squad does not count against his club's MLB Active List, and the player does not accrue MLB Service Time while on the Taxi Squad. The most common reason to recall a player and place him on the Taxi Squad is when a club is considering whether to place a player on the Disabled List but has not yet decided.
Each MLB club is required to roster a minimum of 24 players on its MLB Active List during the MLB regular season (and post-season), but in the event of an unusual or unavoidable circumstance (such as a multi-player trade or multiple trades made at about the same time where several players have been acquired but have not yet arrived), a club's MLB Active List can temporarily go below 24 players, but for no more than 48 hours.
NOTE: Even though an MLB club could play with a 24-man MLB Active Roster for an entire season (and save about $500K in payroll by doing so), in practice and by convention no MLB club would do that unless all clubs did.
No more than 16 players on a club's MLB Reserve List (40-man roster) may be on Optional Assignment at any one time.