Product order quantity

Hello

I need to ensure that customers order a minimum quantity of 12 products (bottles) before checking out.

I know that there are extensions which enable me to do this. But I have an item which is a box containing 12 bottles, so when a customer chooses this item the checkout still counts it as one product.

Is there any extension which will allow me to give a product such as this a value of 12, so the customer is able to check out without having to buy 12 boxes (and therefore 144 bottles!)?