I first came across this in the Marine Corps, which pushes you to ideally no more than 4 direct reports, with the idea that you could have up to 8 directly reporting temporarily, but that would lead to your own personal performance degrading in anything that's not directly managing those 8. Hence the structure would be 4 to a "fire team" with a team leader, 3 teams to a squad with a squad leader, three squads to a platoon with a platoon leader.

There's similar numbers found in the book An Elegant Puzzle: Systems of Engineering Management, and I want to say that similar numbers are espoused by Jocko Willink in his leadership books.