The number on frequency is really irrelevant. My first rating was on a huge procedural sector in Australia that was over 1,000 NM East to West, and over 800 NM North to South (although a lot more than that if you counted the rarely used parts down towards the Antarctic). In that airspace it was normal to have 25-30 aircraft on frequency and it was no problem other than R/T congestion. Whereas I can be doing UAE East sector which is a nasty little 60NM by 60NM sector of crossing arriving, departing and military traffic and be up to my eyeballs with 8-10 aircraft. So as I said aircraft numbers are really an irrelevant figure.
One I would be interested in if they ever recorded it would be average transmissions per minute or something like that as that may be a more accurate indicator of ATCO workload. But then again that doesn't necessarily show traffic complexity.