Commerce minimum quantity and purchase multiplier
Ryan's answer on this question is the approach you'd want to take: 


Install Commerce Custom Product.
Create a new line item type that has new quantity select field and widget. Expose this to the add to cart form.
Hide the regular quantity selection to the add to cart form.
Use a rule to take the value in the field you created in step 2 and set the quantity field to that value on the line item.