Few years ago Gojko Adzic tweeted following:
Even today, several year after the tweet, people do not understand why agile team has to be small. The answer is communication links! Take a look on picture above. The dots are symbol of person in the team, and lines shows communication links in team. Example on the left is for team of five (10 communication links), example on the right for team of nine (36 links). Formula is very simple:
Connection Links=n (n-1) /2n , where n=Number of Team Members
In a team of 15 you will have 105 communication links(15*14/2=105).
Agile is about team, not about isolated knowledge of some individuals.
In trainings I still hear team members complaining about management, who placed timers on each desc, that people time their conversations and do not talk too much. But that is not about communication anymore. It is about company culture. There is a reason why they talk too much, find out what it is, instead of to forbid to talk.