Office stuff is different from carpentry, but I know that one person is less productive (per person) than two, and two less than three. Four is usually overkill. Beyond that, carpentry teams are organized in groups of three when they're not singletons.

I don't know if the sheer number of items on the board is a good measure of productivity. Under certain circumstances, a brainstormer might edit what she says more tightly. I suspect that above a certain number of people, there isn't enough time for everyone to be heard in any case.

I have a hunch that for office environments, the ideal team size is somewhere between 1 and however many co-workers you are forced to endure. I've been at really effective meetings: they aren't "productive" so much as "efficient". Rather than inspiring everyone, they end up with everyone knowing what's been done and what needs to be done, all with the least time spent.

There's certainly a case to be made for fewer meetings that are better run, and smaller teams may make sense, but I'm not convinced that this study has anything to say about introverts vs extroverts.