how do I remove the card number?

To add a new card, follow:

Change, add, or remove Apple ID payment methods - Apple Support

If you have any subscriptions running you cannot remove the card without adding a new one first.

So if you do, make sure you first add the new card, then remove the old card.

Good luck!

You can't choose None as a payment method...

1. If you have an unpaid balance or payment due
2. If you have a subscription that automatically renews
3. If you're using the iTunes Store or App Store for the first time
4. If you are not in the country or region you’re creating an Apple ID for
5. If you changed your country or region
6. If you're a part of Family Sharing

Add a new one first then you can remove the old one.