A standard Xbox 360 game of Halo 3 multiplayer supports four players on one console.
Halo 3 includes local split-screen multiplayer for up to four players, allowing several people to play together on the same Xbox 360. The mode could be combined with system-link or Xbox Live arrangements, where additional players joined from other consoles.
This local limit is separate from the game’s broader online multiplayer capacity. Halo 3 became one of the defining Xbox Live games of its generation, but online and local player counts should not be treated as interchangeable.
The answer is four players for a single-console local multiplayer session. Two-player split screen is also possible, which is why it can be an attractive but incomplete answer to this question.