If you wish to use a product add-on field to set quantity, or you just want to allow a user to purchase products in a quantity of one only, you need to enable the ‘Sold individually’ option. This will remove the standard WooCommerce quantity field from next to the add to cart button.
Click the ‘Inventory’ tab in the ‘Product data’ section:
![](jpg/woocommerce-sold-individually.jpg)