in the add to cart button, set the quantity field to be update able from a select list, then populate the quantity select list with the desired quantity of 6,12, 18 etc.
does the 15% discount apply to all orders across the board?
Set this up on the discounts tab, use the trigger "Number of unique items in cart is > a value of 0
for the calcualtion use Based on multiple of cart subtotal
cart subtotal 'times' '0.15'