Battlefield 3 on Xbox 360 supports 24 players per match (two teams of 12).
This is half the PC/PS3 player count (64 players) due to the Xbox 360's memory/CPU limits. Maps are smaller (no 64-player versions of Caspian Border, Operation Firestorm, etc.).
Game Modes: Conquest, Rush, Team Deathmatch, Squad Deathmatch, Squad Rush — all cap at 24.