Someone_else, keep in mind that even if there are i.e. two sectors active, it doesn't mean that there are two people on the shift. You talk to an executive controller, but there is another one sitting beside him usually called 'planner', or assistant, doing some important coordination tasks. So you need two persons per sector. But they must take some breaks. Even with some minimalistic common-sense break periods (in my opinion anything less than 2hr on/1hr off is simply dangerous), it takes another ATCO to man their positions while they are on their breaks. So we have three of them. Taking into account night conditions and increased fatigue, breaks should be more frequent and longer, so you need another ATCO. So we now have four people for one sector per shift. At some more demanding sectors (and it doesn't mean that there must be zillion aicraft on frequency, it's far more complex issue than just freq occupancy!!) you should include let's say a spare ATCO per two sectors. So, for two sectors at night you should plan 9-10 people.
Pilots should also be aware, that ATC is not only about talking to aircraft, there is A LOT of verbal and computerized coordination involved, which significantly increases the workload.