Direct answer
One player solo, up to four online
1Single-player
2-4Xbox online co-op listing
4Maximum online group size
Official listings
How the stores describe it
SteamSingle-player and online co-op are both listed.View source
XboxSingle player plus online co-op for two to four players.View source
PlayStationOne player locally and support for up to four online players.View source
Scope note
What this does not prove
The store labels confirm the supported group size. They do not, by themselves, confirm every matchmaking rule, invite restriction, save behavior, or cross-platform pairing. Those details should be checked in the current game build.