We play-tested this game A LOT, and this is what we found:
For the main game, the player count is 3-6 players.
Adding the expansion allows for up to 9 players, but can accommodate 12+ (although this starts to become a frantic mess unless ALL players are familiar with the game.)
Multiple expansions can be added to a single game to potentially accommodate more people, but we have not thoroughly play-tested 2 expansions. Our experience during limited play-testing of multiple expansions is that if everyone knows how to play, it's awesome with two expansions!