Once you add a card, your Blockchain.com Wallet will remember it for future purchases. You can manage your payment methods in Settings (under Settings > General in the web wallet or under the main menu in the top left corner of your mobile app). You can add another card, edit the details of saved cards, and delete cards.
Please note that there is currently a lifetime limit of 3 cards (maximum). Removing a card does not reduce the number of cards you've added towards the maximum. Once you've added a card, it will count against this three card limit even if you were to later remove the card. For instance, if you were to add a card, remove that card, and add a new card (or add the same card back), this would count as two cards of the 3 card maximum.