The Discounts Report can be found in the Reports submenu on your event.
The Discounts report provides total coupon and auto discounts redeemed, social discounts redeemed, and how many total orders performed each social share action.
(Note: The Discounts report results include orders where the customer entered a code but didn't use it, and they include any non-discounted tickets on matching orders.)
Below is an example of what you might expect to see on this report, depending on the active promotions on your event.