How many aircraft does an ATC controller usually handle at the same time?
Depends entirely on complexity, rather than sheer numbers. In ATL I can comfortably handle somewhere around 60-70 departures per hour from a ground control position. More than double that when you factor in arrivals. That’s a VERY low complexity situation, with everyone moving one direction. 

On the flip side, I’ve worked TRACON radar positions with five aircraft dodging severe weather and worked my ass off. High complexity. 

ARTCC sectors are different in that they have known numbers, and will go from green to yellow to red depending on saturation. These are based on routings, in trail restrictions and so on. Again, all complexity.