Create a Database and then within that Database, create a Column for the Product Name, Image, Description, etc… then on a Dynamic Page of that Database, add a drop-down box which contains the possible quantities they can buy. I’m fairly certain you can create different payments for different quantities, in fact I found Documentation on this the other day:
https://support.wix.com/en/article/wix-stores-creating-a-physical-product
This pretty much covers what you want.