3-6 sounds like a good rule of thumb.
Research that I have seen hints that people who were in I shaped teams (i.e. single person reporting to a manager) and V shaped teams (2 people reporting to a manager) were much unhappier than larger teams.
Similarly, somewhere close to 10 is when the there is just not enough 'managing' or mentorship happening. I have seen that work only in cases where most of the ICs are senior/staff level and are effectively self-managing.
It arose from the assumption that managers have finite amounts of time and energy to spend with their people and their jobs.
Analyzing British military leaders, Hamilton found that the leaders could not effectively control more than 3-6 people. These figures have been generally accepted as the "rule of thumb" for span of control ever since, at least in the military where the bulk of my management experience lies.