Gears of War: E-Day supports up to 4 players online in campaign co-op, according to the game’s official Xbox listing and recent coverage. It also has 2-player split-screen co-op locally.

Quick Scoop

  • Online co-op: up to 4 players.
  • Split-screen co-op: 2 players.
  • Separate PvE mode: some reporting says Horde Siege supports 12 players total in 3 squads of 4.

So, for the main story online, the answer is 4 players together.