You can settle Catan with up to 6 players. If you want to play the CATAN base game with 5 or 6 players, just add the base game extension.
Please note: If you want to play the expansions with of five or six players, you need, in addition to the base game and the expansion you have chosen, a base game extension and an extension to that expansion - i.e., a total of four game boxes.