This game is played best with 5 players, though the game in its current state supports as few as 3 players. When playing with 3 or 4 players, the game board will be significantly smaller at a size of 4 x 4 map tiles. Additionally, if you play with 3 players, one of the five loot types is not used as well as any cards and islands that produce that type.
All of this information is described in the rule book in great detail.
Hopefully this answered your question!