The official website for The Settlers of Catan

Question about The Cards Town Hall - May I use the Town Hall to choose various cards per turn?

Answer

No. You may only exchange 1 card at the end of your turn, and you only may choose a card in exchange for a card you have placed under a draw stack. Only if you have Odin's Fountain, you may exchange 2 cards.

Note: Odin's Fountain belongs to the Age of Darkness expansion.