The Starter Set can be played by 2 to 4 players, with a few solitaire scenarios also available. By adding a few expansions you can play with up to 8 players.
Lost Battalion Games have put on a game at convention using the Sergeants System with as many as 28 players and 300 soldiers on a side. That was using SMG and included naval gunfire and armor. D-Day Sergeants will add those features to the system in the Fall, assuming we have a successful Kickstarter campaign.