Did you insert a number type add-on in the product? Only in this case you can set the product quantity as the sum of the option values for the number type add-on. How to do it? Enable the Calculate quantity by values amount entry for the number type add-on as shown in the following image.
In this way, the product quantity to add to the cart will be given by the values amount the user inserted in the number type fields.
Remember: you can take advantage of this feature exclusively if there is only one number type add-on among the ones available for your product.