The reason why we hide quantity boxes from "category browse page" is because there wasn't enough space.
If you insist on adding it to category browse page,
then open this PHP file:
(default.php, flexv2.php, defaultlist.php or flexv2list.php... depends on which category browse page you are using currently)
But the layout won't look good. Because the "add to cart" button is big and there is no enough space to include the "quantity" boxes. You will need some CSS works to make things look good. Since it is a custom modification (because quantity boxes are hidden in our DEMO) you are on your own with modifying things and make them look better.
(i would suggest you to decrease the "add to cart" button size little bit)
Use #CategoryBrowsePageQuantity for every CSS rule you add extra, so your new CSS rules will be effective only in category browse page, but has no effect on product details page's quantity boxes.