Can I use multiple Apple gift card to buy an iPhone

Yes, see -> Shopping Help - Payment & Pricing

Payment, Financing, Refunds and VAT - Shopping Help - Apple

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

You can always call 1-800-MY-APPLE and speak to an Apple Specialist for more information.