This rule is analogous to rule 14, using the same logic.
Getting rid of the dropdown list just improves the UX and decreases the number of actions shoppers have to do in order to change the quantity of items they want to purchase.
According to baymard.com most successful stores use buttons for the selection of product quantity. This means shoppers are used to buttons instead of a dropdown list, making this option more intuitive for the shoppers to use.