Sum of Used Credits = Monitored Products + Keywords + Variations in Spy
The total number of credits currently being used can be seen in the account management (via the little black figure in the top right corner):
Credits are used in the following sections:
Monitored Products & Keywords
Under Rankings you can see how many of these credits are currently being used to track (your own) products as well as keyword rankings:
The monitored products can be deleted in the Cockpit. The keywords under Rankings > Keywords.
Spy - 1 Credit per Variation
Monitored products within the Spy also require one credit per Child ASIN. When adding products here, it will automatically add all variations (Child ASINs) of a product.
You can check the amount of variations in the respective column of the Spy overview:
If you delete a product or keyword, its credit will immediately become available to you again.