Does Aniimo have multiplayer?

The cooperative side caps out at four players to a party. That number is the one to plan around, because the co-op content includes challenges built to be taken on with other players rather than alone, and a four-slot party is the group you bring to them.

The competitive side is separate. Aniimo has player-versus-player modes where you battle another player directly, so the multiplayer here is not only people working through shared content together.

The duel arenas

PvP runs through duel arena modes in two sizes. There is a direct 1-on-1 arena, and a 3-on-3 arena for team fights. Both are head-to-head duels rather than open-world skirmishes, so the matchup is the whole event and whatever you have built is what shows up in it.

Not settled: The four-player co-op cap and the 3-on-3 arena are described separately, and it is not settled whether a 3-on-3 team is assembled from a standing co-op party or matched some other way.