This was definitely the wisdom I've heard historically. I suspect the number may be slightly higher than it was 40 years ago due to digital communication, but a lot of what I hear is that it's just really hard to deal with all the personal, and interpersonal problems that crop up.

This also makes me think if you're lucky and have a low drama team, you can do more than 10, but at the end of the day you're going to get a superlinear increase in interpersonal drama the more people you manage so this number is likely to remain small.