The example I gave would allow you to have different multiples for each item.
If you want to use a discount rule instead then add the columns just as I have asked, but don't update the TotalPrice, instead add a new calculation:
TotalDiscount = (([Quantity]>[DiscountMin])?([Quantity]-[DiscountMin])*[DiscountAmount]:0)
Then you can add a discount rule to subtract the total of the TotalDiscount calculation from the cart total.