I was a lead on a test team on the SVC campus and one factor that played an enourmous role in how easy or hard it was to manage my team was how homogenous the task/focus we had was. If I had five people all working on reltatively the same thing, I could gain some effeciencies and more easily manage my group. What I often faced however was five individuals all working on different problems, so that while I only had five ICs, I was managing more like 10 or more projects. This is another problem for another day :)
To your earlier point, there is likely some max number of people that can effectively be said to be “working on the same thing” (i.e. the feature team). What I learned was that if you can get a focused team, you can manage more, but there is still some limit (and in my gut it’s much less than the 30 or 50 you hear quoted at other companies).