Halo: Campaign Evolved supports up to four players in online co-op.
That limit applies to cooperative campaign play across the game’s supported platforms, with cross-platform play and cross-progression also planned. The arrangement lets a full four-person fireteam tackle the remake together rather than restricting online sessions to the original game’s smaller campaign format.
The number is especially notable because the 2001 Halo: Combat Evolved campaign was built around two-player local split-screen co-op on Xbox. Campaign Evolved retains two-player local split-screen on consoles while expanding online cooperation to four participants.
This should not be confused with competitive multiplayer counts from older Halo releases. The original Combat Evolved supported larger local multiplayer matches through split-screen and System Link, but those figures describe versus play, not campaign co-op. Campaign Evolved also omits the original competitive multiplayer focus in favor of its rebuilt story campaign and cooperative features.