I've considered getting one of those mayflash gamecube adapters at one point for games like Screen Cheat (up to 8 player local mp) when my nieces/nephews are over. I already have 3 DS3 controllers paired to a single bluetooth adapter and use a mayflash Wii U Pro adapter (also supports up to 4 controllers) for a Wii U Pro as my "main" controller on PC and local 5 player mp is pretty straightforward. Basically just seems you need to use at least 2 different USB adapters for wireless controllers or I guess just a bunch of controllers wired to individual USB ports.
Edit- I'm a little surprised my posts weren't merged automatically tbh.