Open the code in Shop → Discount Codes and go to its Insights tab. It shows Usages (how many times the code has been used) and Total Applied Amount (the total discount given).
Steps
Go to Shop → Discount Codes.
Find the code. Use the Active, Upcoming, Expired or Archived tab.
Click the code.
Open the Insights tab.
You can also see a quick count on each code's card in the list, for example "12 used".
When a use is given back
If the booking is fully refunded, the use is removed from the count. The player can then use the code again if a usage limit applies.
A partial refund does not give the use back.
A booking that cost £0 after a 100% code is never refunded, so its use stays counted.
Good to know
The Insights tab shows totals only. It does not list which players used the code.
To see the discount on a single payment, open Insights → Transactions and click the payment. The side panel shows the Discount Code.