How do I get my Apple Card account number to set up billl pay?

See this: How to make Apple Card payments - Apple Support

“Bill Pay” is not one of the accepted methods of Apple Card bill payment per your cardhoder agreement.

Billpay from a bank would be handled as a “Non-Conforming” payment.

See “Making Payments” starting on page 11.

https://www.goldmansachs.com/terms-and-conditions/Apple-Card-Customer-Agreement.pdf

See the following from How to make purchases with Apple Card - Apple Support

Pay with your card number

If you need to give a card number when you check out online or in stores where Apple Pay is not accepted, you can find your card number in the Wallet app. If you use Apple Card Family2 and you share your account with a co-owner3, you each have your own card number.

Your card number is different from your physical card number and Device Account Number.