You can view the usage of coupon codes in your dashboard for each booking and as a combined view for each coupon.

Combined View:

You can always see the balance number of coupons remaining on the “Coupons” dashboard in your account.

Per Booking View

If a customer has used a particular coupon code, you will be able to see the same in the booking details for that customer, in your “Bookings” dashboard in your account.