There is a maximum Apple Account Balance of $4999.
Your Apple Account Balance is used for most digital purchases – when buying an app from the App Store, a song from iTunes, paying for in-app purchases, and paying for subscriptions (including Apple Music, iCloud+, or third party subscriptions like Stan, Binge, etc).
You may also be able to use your Apple Account Balance to pay for AppleCare+ on a monthly basis.
The other use for your Apple Account Balance, is to buy physical gear from Apple, including Mac, iPad, iPhone, accessories, or basically anything Apple offer online.
For buying physical hardware (whether it be a new iPhone, iPad, Mac, or other accessories), your Apple Account Balance can be used at;
Apple On Campus (AOC) Education Stores(exclusive online portals with generally bigger discounts than the public Apple Education Store. This link is for the Australian Education Union, but other portals exist e.g. University of South Australia)
Apple EPP (Employee Purchase Program) portals(some employers have an EPP agreement as an employee benefit to get discounts on products often not discounted elsewhere (e.g. iPhones, Apple accessories), but generally have worse discounts on Mac and iPad than the Apple Education Store or AOC Education Stores)
(Apple’s terms and conditions make it clear only eligible individuals can make use of the Education store, the AOC portals, or EPP portals. Apple reserve the right to ask for proof of your eligibility)
If you choose to click and collect an order from an Apple Store – you will need to present photo ID, but this is used to verify that it is you picking your order up.
You can also use Apple Gift Card or an Apple Account Balance when making phone orders by calling 133 MAC (133 622).
You can redeem a maximum of 8 Apple gift cards in one transaction.
You can however, combine this with other payment methods, with the following combinations accepted in one transaction;
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
You cannot use Apple Pay to pay the difference if you redeem Apple Gift Cards or use your Apple Account Balance in the transaction.
An Apple Account Balance can’t be used to make purchases in a physical Apple Store, so plan accordingly.
Apple Store staff are also able to consolidate multiple gift cards; with a maximum balance of $3000 per gift card, this would give you a potential balance of $24,000 if you used 8 cards at once. (via OzBargain)