I think all of the ringmasters are intended to be one and the same and the Defeat all Ringmasters marvel is just a short way to express completing the four main Ringmaster battles. The artwork and voice acting does not seem to indicate multiple ringmasters, same with the wizard mode. I think the Ringmaster Battle can be repeated as long as the player doesn't complete all of the marvels, so it doesn't really make sense that there could be an unspecified number of different ringmasters. Finally, a circus typically only has one person in charge, the ringmaster.