> I've managed from 2 (and was therefore an IC) to ~40 people.

~40? I'm assuming you're counting your skips. If so, a better word to convey what you meant would have been to say that you were directing them, to indicate you had skips and were actually managing a half-dozen or so managers. If not, how does one manage to keep track of ~40 relationships (plus other colleagues) in a 4\b80h week?