At the moment, it's two.
However the rules are designed to be expandable to multiple players (and I've tested it that way) if you have differently colored player counters. I didn't make the rules notations for "if playing with more than two" players or include different colored counter sheets. I will if there's a demand for them.
I guess my thinking eventually was super-ultimately to sell the game as a two player and then sell the 3rd and 4th players as separate add-ons.
I suppose i could add a new re-colored player counter sheet and add two more players?
Simply stating in the rules that the game is intended for two players would be sufficient for me. An expected playing time would also be useful.