Monitored how? For what?
We have several central stations where operators are "monitoring" 1000+ plus of our cameras at a time (camera:operator ratio of 1000:1). However, they are using the video analytics to call operator attention to particular cameras, and the cameras are (relatively speaking) concentrating on a fairly narrow task.
If you're talking about live views, the numbers I've always seen are in the 20-30 range, and various SOC's and CS's I've visited seem to indicate that 20-30 is the "typical" number. In those cases, operators are usually not staring at the cameras continuously, they are doing other taks. This, IMO, *increases* the total amount of time they can spend monitoring the cameras, but also *decreases* the probability that they provide near 100% "coverage".
To be fair, relying on analytics (or any other non-human method) can also add the risk of a missed event.
So, I would say, 1 operator could manage 24 cameras with near 100% effectiveness for about 30 minutes in a live view scenario. Or, they could monitor 24-48 cameras at ~75% effectiveness for 2 hours (basically by time slicing among multiple tasks), or 48+ cameras at 50% effectiveness for 8 hours by time-slicing and camera-slicing (eg: viewing different groups of cameras in between other tasks). This is assuming you're looking for macro-level things:
1) Obvious movement in the scene
2) Obvious scene failures (out of focus, poor lighitng, etc)
3) Presence/non-presence of key items (trucks, inventory, gate open/closed).
If you're looking for minor-things (counting to ensure all 28 forklifts are parked properly, ensuring locks are in place, ensuring that a door is not slightly propped open) then it would take more resources to cover the same number of cameras, or less cameras per operator.