In The Mythical Man Month, Fred Brooks points out that in a team of n members there are n(n-1)/2 potential links, leading to an n-squared scaling with team size.
A team of 15 has 50 percent more links than a team of 12, more than twice the links of a team of 10, and five times as many links as a team of seven. This is broadly consistent with the conclusions previously discussed under The maximum team size for effective working.
Time is limited, and each relationship requires time for maintenance. Because of this, it is practical to keep working groups small. Some claim the sweet spot is around a team as small as seven. For our purposes, we will place the practical upper limit at 12, based on an observed sociological phenomenon known as an empathy group . Larger teams can exist, but they will usually factor into sub-teams.
From an article in CrossTalk: The Journal of Defense Software Engineering.