since some of cupcakes are sold by the Doze, and some are sold as singles, this is going to be tough. the simple way around this is to not allow the quantity to be editable. Remove the quantity text box and replace it with the the static quantity binding from the bindings panel.
If you want to allow the quantity to be editable, you will need to add a new column to the cart object for type. In the add to cart button set it to "dozen" or "single".
then create a new calculation on the calculations tab:
(abs([type] == 'single' && [Quantity] < '6')?[Quantity] = '6':'') || (abs([type] == 'dozen' && [Quantity]%12 >0)?[Quantity] = ((ceil([Quantity]/12))*12):'')
the firsst part of the calculation will make sure that the single type is always more than 6, the second part makes sure that the dozen type is always a divisor of 12.