If all the items are in the same category you can set the discount to apply to matched items them the entire item in that category will qualify for the discount. Go To Marketing > Discounts > Discount Methods and edit the discount method in question (or create a new one) 
If they are not in the single category you will need to have an item in cart rule for item number, or item in cart (multiplied) if you need it to apply to more than one of the same item. At this point you will more than likely want to make the discount not strict otherwise the customer will have to have all the item in the discount method in there cart before the discount will apply.
