Quantity Edits
Enable customers to increase the quantity of items purchased and pay the outstanding invoice to update their order. Only if you have order downsizing enabled will customers be able to decrease the quantity of items purchased, in which case a gift card will be sent to the customer.
Customers will be able to increase the quantity of items purchased if there is stock available in the products Shopify inventory. Only if you have chosen to ignore inventory levels will customers be able to exceed the amount of inventory that's available.