Correct. Heck even 2 people let's say mc and usagi show up it's enough. Even if the other 17 die leaving mc, usagi and j <3 behind the game will end. It's simply not possible unless everyone is a stranger.kinda semi plot hole that all participate are strangers to eachother. If any premade group showed up they would absolutely dominate (like the MC's group for example) it would take literally ages for them to betray eachother.