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.