Max Amount of Apple Gifts Cards on an Apple Purchase (In-person versus Online)?

It does not appear a limit exists, per: Legal - iTunes Gift Cards and Codes - Apple

Though… You agree to not use Gift Cards or content codes in any manner that is unlawful, misleading, deceptive, unfair, or otherwise harmful to Issuer, Apple, or its customers. We and Apple reserve the right, without notice to you, to void or deactivate Gift Cards or content codes without a refund, suspend or terminate Accounts, request alternative forms of payment, suspend or terminate the ability to purchase Products or to purchase or use Content, cancel or limit orders, and bill alternative forms of payment if we suspect Gift Cards or content codes were obtained, used, or associated with an Account unlawfully, fraudulently, or otherwise in violation of these Terms.

Apple lists a “for assistance “ telephone number in the footer here Apple Gift Card - Apple — check with Apple directly lest you end up disappointed.

The vendor may conceivably limit purchases, as well.