Every Scheduled Event will record Occurrence Counts once that Schedule has launched a Dispatch. To see the Occurrence Count for any Schedule, just open that event and then click on 'VIEW HISTORY'. From the time the System first launches a Dispatch for any Scheduled Event, the Occurrence Counts will be part of that history.
The first time the schedule launches a Dispatch, the Occurrence Count is 1. Each subsequent time a new Dispatch is launched for this schedule, the Occurrence Count increments by 1, so the second time this event launched a Dispatch, the Occurrence Count was 2, and so on. The Occurrence Count is literally the number of times that Scheduled Event has launched a Dispatch.
In the screenshot sample scheduled event below, you will see that the Daily 1st Round Inspection event has launched a new Dispatch 36 times as of this printing. Hence, the Occurrence Count incremented from 35 to 36.