What is the maximum number of credit cards that I can use with Apple Pay?

Hi

Up to 12 payment cards per device can be added to Apple Pay on Apple Watch Series 3 models, iPhone X, iPhone 8 and iPhone 8 Plus.

Up to eight cards per device can be added to Apple Pay on other eligible devices.

More information:

Apple Pay is compatible with these devices - Apple Support

Set up Apple Pay - Apple Support

The information is confirmed in the first part of the support article linked in my previous reply:

Set up Apple Pay - Apple Support