If you coupon says save $ when you buy 1 you would need one coupon for every one you bought.  If you coupon says save $ when you buy 2 you would need one coupon for every two you bought.

If the coupon says one per customer than that is what it is.  You can use another one the next time you go to the store.

If a coupon has a limit of how many like coupons that is the maximum number you can use that time you can use more the next time you go to the store.

If a coupon says one per purchase than you can use one for each one you are buying as long as it is $/1 item if it is $/2 than one for every 2 and so on.