How many people fit in one game?
One to six players, including the host. The host can invite up to five Steam friends. The store and developer FAQ also confirm that you can play alone.
“Player count” can also mean Steam’s live concurrent-user statistic. That number is not the capacity of your lobby. Mods that expand a lobby do not change the unmodified game’s stated limit.
Invite your Steam friends
This is a Steam-friends-only game, with no random matchmaking. Use Steam invitations to bring friends into the session.
Lobby Mode offers Friends Only and Invite Only. To keep uninvited friends from joining, the developer says everyone should set Invite Only, then the host should create the lobby again. The setting is beside the player profile on the main menu, or under Settings > General > Lobby Mode.
Invite Only is a privacy setting, not a verified cure for all connection errors. Official platform support is Windows PC on Steam; a Steam Deck playability rating does not establish every controller input.