Can I use multiple prepaid Visa debit cards to purchase an iPhone 17 Pro?

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

"You can use up to eight Apple Gift Cards at a time when you purchase online from Apple, or if you’re placing an order by calling 1‑800‑MY‑APPLE. You can use your gift card(s) with one credit card and your Apple Account Balance to pay for your purchase."

In store you should be able to do that yes. But make sure you tell the Genius person helping you, that you plan on paying with 9 Visa cards so they are aware.