Klei believes that Don't Starve Together will "support 2-4 players simultaneously. Could be more, but we are aiming for 4 minimum." The expansion will be free for all current Steam or standalone PC players to download, though the price of the standard game will increase by $5 (to $19.99) once the alpha version of Together launches. Meanwhile, Klei just launched its single-player Reign of Giants DLC.
Update: When asked about multiplayer support for the game's PS4 version, Klei tweeted that it would "like to do PS4 as well, but it's far away from knowing if it's possible."