What is the shop's max purchase quantity?

The cap is per transaction: up to 100 of one item at once, where the old limit stopped at 99.

It only matters for goods that have no tighter limit of their own, and many shop items do. Breezy Mission Letters in the Legendary Journey Shop are limited to five a week. The Mysterious Vendor carries only a limited stock of growth fruits. Voxel Coin spending in the Astra shops runs into a weekly coin cap of 1,000. In all of those cases you hit the item's own limit long before the purchase cap.

Where the cap does come into play is bulk buying of cheap, unrestricted stock. Aniimo food in the homeland weekly shop, such as Growth Tonic and Energizing Workmeal at 50 Glimmer each, is one example. If you need more than 100 of something, you buy it in more than one transaction, as long as your currency covers it.