You can set the discount to apply to matched items if all the items are in the same category. Do this by using the Category in Cart rule.
If they are not in the single category you will need to have one item in cart rule for each item number. Or item in cart (multiplied) if you need it to apply to more than one of the same it. At this point, you will more than likely not want to make the discount strict. Otherwise the customer will need all the items from the discount method of their cart before the discount will apply.
