Decades of research suggest the sweetspot may be between 4 and 6, as reported in a recent Fortune article: How to build a great team. But it's not that simple: writers uniformly remind us that other considerations contribute to team success and influence the "correct" number for a given context.
Some of the other factors mentioned are:
- clarity of team goals
- type of team - function, domain
- composition - roles required
- team process
- whether roles are clearly understood
- presence of an outside threat
- observance of team norms