http://www.ign.com/articles/2018/06/10/forza-horizon-4-every-season-is-racing-season-ign-first
3ページに
The biggest change is the way Forza Horizon 4 is being built as a shared world game,
where the AI-powered Drivatars of Forza Horizon 2 and 3 are being replaced by
real players within a 72-player server.
4ページに
Forzathon Live supports 12 players
とある