If leader should be teacher, then...

If we follow the leader = teacher concept, then the effective number of direct reports should be similar to the effective size of a classroom.

Jon Miller writes about The Lean Workplace as Classroom:

Teacher student ratio. Classrooms with more than 12-14 students per teacher do not perform as well as smaller class sizes. In the workplace this is known as span of control and likewise when a team leader has a team size that is too large, quality, safety, productivity all suffer. Too many companies look only at the indirect to direct ratio and increase team size, reducing one small visible cost while increasing other large, less visible ones.