>>This means I will have a card with a different number when I check in. What should I do?<<
Nothing. Assuming you've paid everything upfront (and not the 50% deposit), Airbnb has your cash and will pay the host 24H after check in. If you've only paid the deposit, you'll get a message from Airbnb if they can't debit your card (b/c the number has changed).
Your host does to need to see your credit card, @Linnea19.