Final answer:
The maximum capacity of an M9 pistol magazine is 15 rounds, although there are extended magazines with a greater capacity.
Step-by-step explanation:
The maximum capacity of the magazine for an M9 pistol, which is a standard sidearm used by the U.S. military and other law enforcement agencies, is typically 15 rounds. The M9 is a 9mm semi-automatic pistol that has been in service since the 1980s and is known for its reliability and ease of use. While the standard magazine capacity is 15 rounds, there are extended magazines that may hold more; however, for most standard applications and according to military specifications, the capacity remains at 15 rounds.