Forza Horizon 3’s online co-op mode can be joined by four players.
The four-player limit applies to the cooperative campaign, where a host leads friends through Horizon Festival events. Participants can play the campaign together, and their progress is designed to count for the players involved rather than treating co-op as a disconnected side activity.
This feature was a major addition to the Horizon formula. The game supports cross-play between Xbox One and Windows 10, so eligible players on those platforms could join the same cooperative session. The design also aimed to reduce practical barriers: players could take part even when they did not own identical cars or have matching progression.
The frequent mix-up is with Forza Horizon 3’s competitive online modes. Those modes could support up to twelve players, but that is not the cooperative campaign limit. Four means the total co-op party, or one host plus up to three friends—not four additional players joining a host.