A standard Among Us game needs at least 4 players.
The four-player minimum is the smallest lobby size supported for the ordinary multiplayer game. Even at that size, the basic structure is present: Crewmates work through tasks while one or more Impostors attempt to eliminate them and avoid detection. Players use movement, alibis, emergency meetings, and voting to decide whom to eject.
Four players does not mean every possible setup is equally balanced. The host can adjust the number of Impostors, task rules, kill cooldowns, player speed, and special roles. Those settings can make a small lobby feel very different from a larger one, and some custom modes or community rules may impose their own requirements.
The minimum is often confused with older player-count limits or with recommended group sizes. Among Us later expanded its maximum lobby size, but the lower limit remained four for the standard game. The official game description commonly presents the supported range as four to fifteen players.