The official website for The Settlers of Catan

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Play The Settlers of Catan with 5 or 6 Players

You can settle Catan with up to 6 players. If you want to play the Base Game with 5 or 6 players, just add the Base Game extension.

However, to play the Base Game expansions with 5-6 players you need both the Base Game extension and the extension of the corresponding expansion. For example, if you want to play the scenarios of the Seafarers Expansion with 5 or 6 players, you need one Base Game extension and one Seafarers extension.