I've been building up Gamecubes for 4 player 4-Swards Adventures.
I currently have 2 NGCs and 3 GBPs with discs.
I did an experiment, it seems you only need one GBP disc, because you can boot up one Gamecube, take the disc out, boot up another, Take the disc out, etc, etc.
More on topic,
Bomberman Saturn has 10 player support. And apparently has online support for a Saturn modem.
There is also always Minecraft
