Two-pizza-size Team

Two-pizza-size team - is the term for optimal team size. “If you can’t feed a team with two large pizzas, it’s too large.” - Jeff Bezos (Amazon Founder). According to Jeff Bezos rule, an optimal team size should be no more than 8-9 persons.

Scientific research in the psychology field also support this rule and shows that the bigger the team the less productive it is. 

Besides, there are more links in a bigger team between the team members. 
The number of links = team-size(team-size-1)*2 

So, there are 112 links between people in an eight-person team and 180 links in a ten-persons team.

See also Dedicated Team (Staff Augmentation)Outsourcing.