In my Prestashop website:
The field 'Select a quantity' should increase the number of pieces in multiples of the minimum quantity purchasable for that product.
E.g.= The product named "Blue pen" has the minimum quantity purchasable of 8 pieces (it is set up in this way in product details). If the customer click on the up arrow to increase the quantity he will see 16, then 24, 32, 40 etc. and he can't have the chance to order a different quantity such as 10 pieces, for example.
This criterion should work in every order review page, in the cart and in the main order page, so wherever you have the chance to change the quantity of the products.
It should work for 2 Prestashop sites, both in the same domain.