You can access the Class Inventory Report from the Menu drop-down. Reports > Finance Reports > Class Inventory Report. The Class Inventory report shows a snapshot of the remaining inventory you have left for your products/classes. In the image below, you'll see that I entered the Sku for the product that I want to see the inventory. You can leave the Sku filter box empty and all of your classes/products will be found in one report.
There are several things to keep in mind for this report to be operative. You need to have inventory enabled either at the product or variant level. You also must have a Class Start Date selected as shown below.
This report is handy because it can show you inventory for all classes on one report if you don't enter a specific Sku. If you just want to find the inventory of one class, you might find it faster to just open the product in Catalog > Browse to check your inventory. Reserved seat ticketing has been changed so they don't appear on the report to enable you to find your classes more easily.