The general answer is 3 cards. But because some people have thicker cards, we can't guarantee an exact number of cards that will fit for every user. Here's what we can tell you:
iPhone 13 case models: These cases easily fit three credit cards of mostly any thickness. In some instances, you will even be able to fit four cards. That's because we made the card slot a little deeper with these newer models.
iPhone 12 case models and older: These cases will usually fit three credit cards with ease, but some users with thicker cards might only be able to fit two.