Want to sell digital goods on Opencart 3.0 - then you need the Quantity "1" module in the cart. After applying this module, the customer will be able to add products to the basket only in the amount of 1. Even if the buyer adds one product to the basket several times, the basket will still contain the amount of this product 1. Also, the buyer will not be able to change the number of goods on the basket page, placing an order - it will always be equal to one.
Selling products in one copy is suitable for information products since it makes no sense to sell one file to a client in the amount of 2 or more. Selling goods in only one piece can be useful in other cases - for example, if you sell very rare goods, such as antiques, and coins. Or you want to sell one copy of the product in one hand.
When changing the number of goods in the cart or on the checkout page, the module will not allow to increase it by more than 1.