Depending on who you ask, the perfect group size is variously 7 ±2, or 8 to 12, or 3 or 5 people (”companies that make the INC 500, a list selected for extraordinary five-year growth rates, tend disproportionately to be founded with 3 or 5 employees, rather than 4 or 6.” - Dean Brooks, comment #10 on the same post). I say prime numbers work best, because they don’t divide.