This info is currently found on both the Australia and German eShop listings for ARMS. Nintendo is yet to say anything about 8-player support so far. Either this listing is incorrect, or it's info that accidentally got shared before it was meant to!
I'm so excited for this game. Me and my sis had a blast playing it at Chicago's Switch event. If it has online multiplayer of that capacity, it is going to be amazing. She destroyed me!
I'm very intrigued how the game will work with eight players at the same time. Could be tournament setup but would't make sense if it says 2 to 8 online or 2 to 8 with two switch connected (wireless mode).
I imagine this is more a tournament or lobby-related thing. There is almost no way this game will support 8 fighters.
Or there is a minigame, like the one in single player where you punch targets.
Ha! It would be funny if there was a segment the Smash one, where the announcer kept upping the amount of players playable.