How do I get the whole Apple Pay card number from my default card

There is no whole number for it. You can get the last 4 digits for refunds or disputes.

If you need to enter a full 16 digit number in a website or other location that does not support Apple Pay, you can use the virtual card number. This virtual card number will not work for refunds or disputes regarding purchases using Apple Pay in physical store and merchants.

click here ➜ How to find the card numbers associated with your Apple Card - Apple Support