Four people is the optimum sized team for collaborating on a project.
This is far from scientific and it’s only one example but it’s interesting. In this case it seems to imply that for work that relies on generating activity (e.g. generating ideas, editing existing content) 2 or 3 collaborators on a project are better than 1, and 4 are better than 1, 2, or 3, but 4 are also better than 5, 6, 7, 8 or 9. Not until you get to 10 or more collaborators do you see an increase in activity per person. This makes some sense as collaborating on a project sparks more ideas, if you’re accountable to a group of people then you’re more likely to hit deadlines, but once you get above 4 people the extra communication needed to coordinate works against individual productivity.
Once you hit 10 or more people a different communication protocol emerges. One of two things is probably happening. This is probably closer to crowdsourcing than collaboration. You probably have a few owners of the project with multiple inputing only occasionally.
There’s probably other research out there on optimal team size, I should find some.