Research shows that the optimum team size is five (actual research is 4.6). Team effectiveness and cohesiveness drop at six. Teams tend to be overstaffed, often because managers don't want to leave anyone out.
The more of a head start your organization gets in developing people individually and as teams, the more difficult it will be for competitors to ever catch up to you.
Trust is the most fundamental element of a winning team. If people think their teammates are lying, withholding information, or plotting against them, nothing valuable will be achieved.