Just tested SIMULTANEOUSLY with:
* 2 wireless PS4 controllers
* 2 wired PS4 controllers
* 2 wireless XBOX 360 controllers
* 2 wired XBOX One controllers
* 4 wiimotes (mayflash setting 3)
* 2 wired switch pro controllers
* 1 wireless steam controller
* 1 keyboard

NOTHING IN LIFE WILL MAKE YOU HAPPIER THAN 16 PLAYER BLOCKPOOPER. If you are having carnal relations with 15 other people at this very moment, I recommend you play BP9 instead.

There was a poor configuration choice on my end that made this difficult previously. Special thanks to supersmo who brought the option to my attention!