Options with predefined product quantity

I would like to create a product in my open cart site in which, the options need to like this :

Options :
1. Pack of 12
2. Pack of 14
3. Pack of 20

Each option have a unique pricing, and the product can't be sold loose.
ie; it is lot product.
So when a customer buys a product the quantity of stock reduced, and total cart price needs to according to options.

Is there any way to do this in open cart, or any extensions available?
Can anybody please help?

Why not set the product up as a pack of 12

Then when someone buys 1 quantity they know they are getting 12 items in the pack.

Divide your stock value in admin by 12

No need for options then either as they can simply insert the quantity and get that number X packs of 12

As above or it could be done with options easily enough.