Probably this is a good feature, because I have found many threads to this topic. But how the hell you set this on disabled as standard in the template settings? For everyone who is fighting with this problem: Go to the Quark template, tab "Features",scroll down to "Settings for VirtueMart 3 only - Show quantity fields in the product details page" and choose "Enabled"! Now you can see the quantity box at the product details page.
I hope this will help some people!