will this minimum quantity apply to this one item or others?
you could set a minimum quantity on a product by product basis by adding a new column to the cart and naming it minimumQuantity
in the add to cart button behavior set this column to the minimum you would like customers to purchase for that item, themn create a new calculation as:
abs([Quantity] < [minimumQuantity])?[Quantity] = [minimumQuantity]:""
that will force the Quantity to be the minimum set for that product if they try to set it below the minimum allowed.