GetPaid lets you add a quantity selector for items on the checkout forms, to help your users specify quantities of certain items. 

How to Add a Qty Selector to Checkout Forms?

If you only need to add the quantity selector for an individual item, the best way to do so is to specify 0 as the quantity when adding a Buy button or displaying a form: 

[getpaid item=2120|0]
[getpaid item=2120|0 button='Buy Now']

However, to display quantities on the form, you will need to click on the “Item totals will appear here”, then open the item, and check the “Allow users to buy several items” checkbox.

This will allow users to specify quantities of individual items.