What is my Apple Pay number?

The refund will be returned into whatever your default payment card is for Apple Pay, into the selected card in Apple Pay if you picked one to pay with, or into Apple Cash if that was were you selected to pay from.

It’s this process used when purchasing, with the transfer from that payment reversed: Make purchases using Apple Pay - Apple Support

I don’t think Apple Cash has an account number, as Apple Cash uses your Apple ID.

Apple Pay uses your existing debit and/or credit cards, and does not have its own number.

Apple Card has a virtual card number, and which is visible in the Wallet app.