In the app, under Profile → Payment methods. That's where you add a new card, see the ones you have saved and delete the ones you don't use. You no longer need to write to support.
How many can I have
Up to 4 per country, and each country has its own wallet: payments in each market are processed by a different provider, so cards don't mix. When you browse cars in another country, the app switches to that country's wallet.
Does the card have to be from the country where I rent?
No. It can be an international card from any country: if you live in Uruguay and you're going to rent in Argentina, you add that same card to the Argentina wallet. All you need is to add it to the wallet of the country where you'll rent.
Which ones are accepted
Credit and debit, as long as they're international. Prepaid cards are not accepted.
Which one is used to pay
The last one you add becomes the default. You can change it at checkout before confirming, and the trip is tied to that card: the security deposit is held there and the refund goes back to that same card.
When a card can't be deleted
it has an active trip;
the trip ended less than 30 days ago;
a security deposit hasn't been released in the last 30 days;
it has payments still being processed.
If it has no activity, it can always be deleted. You can end up with no saved cards, but you won't be able to book until you add one.
If a payment or a deposit is declined
We let you know by push and email which card it was. Open the trip, pick another card and try again, as many times as you need.
Which cards are accepted in each country is in What payment methods does TripWip accept?.