The official website for The Settlers of Catan

Question about The Cards Settlements - If all settlements are built and already some cities too, can I continue building settlements by using the settlements that are placed under the cities?

Answer

No. The total number of localities you can build (Metropolises, settlements, cities, and Citadels) is limited to 9.

Note: Metropolises and Citadels are part of the Catan Card Game – Expansion Set.