How many Apple gift cards can you use for one transaction?

Up to 8. See the following from Payment, Financing, Refunds and VAT - Shopping Help - Apple

Using more than one payment method

You can also combine payment methods to pay for your purchase. Choose from the following combinations of credit cards and gift cards:

• Credit card(s) - up to two
• Apple Gift Card(s) - up to eight
• Apple Gift Card(s) - up to eight + one credit card
• Apple Gift Card(s) - up to eight + Apple Account Balance
• Apple Gift Card(s) - up to eight + Apple Account Balance + one credit card
• Apple Account Balance + one credit card

Apple Gift Cards and Apple Account Balance cannot be used with Apple Pay, PayPal, or split credit card payments. Please note that you can use the value from all types and generations of Apple gift cards. For more details, see:

• How to use Apple Gift Cards and App Store & iTunes Gift Cards
• How to use Apple Store Gift Cards

During checkout, if you are paying with a gift card issued by a credit card company, you can combine it with one credit card.