The quantity box normally appears on the category template if there are no variants for the product...if there are attributes set up, then you won't see quantity box until you get to the product page.
I've just checked it on a new site I'm building now using 3.90 and there is no quantity box on it there either...weird.
Older versions of nopCommerce do have the quantity box on catalog pages if there are no variants...I'm not sure why that's been removed.
We might be able to add it back in to the /Views/Shared/_ProductBox.cshtml somehow...I'm investigating.
*** Okay...evidently it's just that on the older sites I am using custom themes which have a plugin called
AjaxCart. It DOES have the quantity box appearing in the CategoryTemplate.ProductsInGridOrLines.cshtml view.
I guess we'll either have to do some custom work, or buy that plugin.
I can say that the nop-templates AjaxCart and QuickView plugins work very well together.