Player-count answer
You can complete Crimson Moon alone or with one online partner. Steam lists both single-player and online co-op, while the developer documentation places matchmaking at the portal in the Sanctus Clypeus hub.
The published feature set does not list couch co-op, split screen, or groups larger than two. “Multiplayer” on the store therefore means the optional two-player online mode, not a shared-screen or large-party system.
How co-op fits a hunt
Both players prepare loadouts before entering a mission. The official build explainer says partners can coordinate equipment and temporary Boons rather than being forced into fixed classes. Developer support material also describes shared-life and revival interactions, with difficulty adapting to cooperative play.
Launch caution
The supported mode and the quality of a particular session are different questions. The September 1 known-issues list documented invitation, disconnect, progress-award, and client-freeze cases. Those reports do not remove co-op support, but they make the current technical guide worth checking before a long session.