Basically, "occupancy" is a number of aircraft at the same time in a sector. In a way, that's the number of aircraft on the sector's frequency at the same time.
You can also have aircraft in your sector, not on your frequency. Were they "expected" (by CFMU) in your sector or not ? You can also have aircraft on your frequency, outside your sector...
What's the purpose of your question ?
Do you wish to know how to "calculate" an "occupancy" ?