One way you could do this would be to include the minimum quantity value for the item when it is added to the cart, you must ensure that you have a column in your cart to hold this value.
Once you have done that you can create a new calculation that will help you enforce the minimum quantity. In the calculations tab create a new calculation and set it up like this
([Quantity] < [MinQuantity])?[Quantity] = [MinQuantity]:[Quantity] = [Quantity]
This calculation assumes that your new column is called MinQuantity, the name of the calculation should not matter. What it is doing is comparing the value of the Quantity and MinQuantity columns, if the Quantity is less than the MinQuantity then the Quantity will be set to the MinQuantity, otherwise the quantity will remain the same.
The other part of this would be to make sure that you default the quantity field to the min quantity in your add to cart form.
