= a Group Tresholds number
7, "The Working Group". This community size probably runs from about 4-9 members, but 7 is a pretty good average, and one that shows up in multiple studies. This number may well relate to the general rule of seven (original paper), which suggests that 7 is a number that the brain can easily and intuitively comprehend.
It has become increasingly clear that a tightly-knit group of 7 is the first group size which is truly an optimal community size. Groups below this size can function effectively, but risk not having enough manpower to deliver a result that everyone is happy with, or having insufficient viewpoints to avoid group think.
Seven is not only an optimal size for a wide variety of corporate and government committees, it is also a healthy size for a small business and even a good size for a party of close friends. More importantly, 7 is a very comfortable group size as it "feels" relatively natural. At this size members find it easy to get to know the other members of the group, and they're able to function well together in a very intuitive and organic fashion.
Squad and Fire Team An interesting example of this group size is the modern infantry "squad", which consists of two fire teams of 4 people, and a squad leader, for a total of 9 people. Each fire team is is large enough to function on its own, but together the group of 9 can still have effective small group dynamics.
It is typically at this size that the first signs of leadership in a group informally emerge, but the leadership usually isn't overbearing at this level, nor does there tend to be any rebellion against it — perhaps because the group may be too small to elicit multiple leaders.' (http://www.lifewithalacrity.com/2008/09/group-threshold.html)