The 6 player mode is available on Switch, Xbox and PC, but on PS4 (same thing with the PS5) you will unfortunately be limited to 4 players.
That's simply because the console allows up to 4 controllers simultaneously.
However, you can use the 6 player mode online as long as each player is connected to their console ( for example, the mix of 4 local players and 2 online players doesn't work).