multiplayer compatibility between steam and itch is very nice.
How may players can there be in a game? Can you freely drop in and out?
In theory 127 players, but I would not recommend more than about 5. Players can join or leave at any point during a game.
The multiplayer is cooperative, but there is no ingame chat so it is best played with friends using a separate communications tool. (There are also not many players so do expect to have to organise a group to get started.)