How can I view my full Amex card number on Apple Pay?

Apple does not store your full card number. You will only be able to see your last 4. The full card numbers are not used for Apple Pay.

Apple Pay is also designed to protect your personal information. Apple doesn’t store or have access to the original credit, debit, or prepaid card numbers that you use with Apple Pay. And when you use Apple Pay with credit, debit, or prepaid cards, Apple doesn't retain any transaction information that can be tied back to you. Your transactions stay between you, the merchant or developer, and your bank or card issuer.

Apple Pay security and privacy overview - Apple Support

Why do you need the full card number, for a refund? If a merchant is telling you that, the clerk is wrong. Escalate the issue to store management or district manager. Federal banking regulations say last 4 is all that’s required. Their payment network operator (American Express) will confirm.