Hi,
As long as the "Display 'add to cart' button for free products" setting is activated in the HikaShop configuration, the quantity field will display regardless of whether the product has a price or not.
There is not such option per category.
You would have to create a custom category field to turn on or off that option per category by editing the file "listing" of the view "product" and adding such code at the top:
<?php $this->config->set('show_quantity_field', $this->element->XXX); ?>
where XXX is the column name of your custom field. The value needs to be 1 (one per product) or 2 (global).