The maximum number of human players in a standard Mario Party game is four. The traditional board mode is built around four characters moving through a board and competing in minigames at the end of each turn.
If fewer than four people are available, the remaining characters are controlled by the game as computer players, commonly called CPUs. This keeps the board structure and minigame formats intact while allowing one, two, or three people to play together locally.
The four-player design supports several types of minigames. In free-for-all events, all four contestants compete individually. In 2-vs.-2 games, players form teams, while 1-vs.-3 games divide the group into uneven sides. These formats are a major reason the series traditionally uses four active characters in its standard mode.
Some later Mario Party-related modes and spin-offs have experimented with different multiplayer arrangements, including modes that can involve more characters or online participants. Those exceptions can cause confusion, but they do not change the standard board-game limit. In the classic format, four is the maximum number of human players.