In my experience it takes six or more people to fully occupy a manager. Ten seems to be about perfect. You can go higher than that (and many do) but at a certain point you’re not evenly investing in all your people anymore, you’re mostly focusing on a few at a time.
Obviously the hardest part of this is if you don’t have 6+ people to manage. My answer to very small teams is not to have a manager at all, just have a technical lead and trust that a group of 1-5 people can work out their own crap.