Some anthropologists have speculated that groups of 6 to 9 people come from our long evolutionary heritage as hunter-gatherers, and that such group sizes made for effective hunting bands.
Evolution tends to be quite efficient over time at finding balances between trade-offs.
So it is likely that working group sizes of 6 to 9 people evolved because they represent a balance between the benefits of scale (a hunting band can get more food per calories expended than a lone individual can), and the diseconomies of complexity. Our ancestors would not have survived for long if hunting groups of thirty people spent hours debating whether they should hunt bison or antelope that day.
— The Origin of Wealth by Eric D. Beinhocker