Here's a quick guide if anyone would want to run pico park with 8 players in local mode just like we did during Speedons.

1. Get a powered USB hub. The one i am using : https://amzn.eu/d/03VxhAjF Why powered ? If you plug many usb controllers on the same hub, it might have issues recognizing or powering the different controllers. I even encountered a weird issue where the inputs were mixed up ! We had 7 USB controllers on the USB hub, and one person with controller/keyboard directly on the computer.
2. In steam settings, install the xbox controller drivers for extended support. https://images.steamusercontent.com/ugc/2012593630419989216/C1FE814A0D119E67EF7736BD0DC90E34773E0833/ This is a specific driver that allows steam to manage up to 8 controllers at the same time. If it is not installed, the game will only let player 1 to 4 use controllers.
3. If you want/need people to use keyboards, you can usem ultiple keyboards too ! In Pico park settings, make sure they keyboards binds are different for each player. Then, plug multiple keyboards. We tried with one player using WASD and another using the numpad numbers on 2 different keyboards.

And that's about it ! If you have any questions, feel free to ask.