If it's like the other mario party games on ds and 3ds, I think you only need one copy. Mario Party: Star Rush also has a version of the game called Mario Party: Star Rush - Party Guest. This app can be downloaded off the Nintendo eShop for free. While it can be played alone with very restricted minigames, the primary focus of the app is to play full multiplayer with other players with only one game cartridge. If players buy an official copy of Mario Party: Star Rush, players can transfer saved data from Mario Party: Star Rush - Party Guest to their copy of the game.