You can sell as many items as you like with the Sell Online. There is no monthly minimum or maximum. However, you can only sell one item per custom button. There is currently no 'quantity' feature within the button. For example, if you own a spa your customer can only buy one pedicure, not a pedicure and then go back and add a massage during checkout. The massage would have to be purchased separately through a different custom 'Buy Now' button, or you can bundle them together as a package deal.