We know teamwork can have its advantages and disadvantages depending on the members of the group. Too many people suggesting too many ideas isn't ideal. They might end up not starting anything. How many writers are good enough so that it won't be too chaotic?
I think it's the personalities that matter more than the numbers. You need a group that can work together and play off one another, but aren't identical. I bet it's hard to find the right sweet spot.