In YC experience, 2 or 3 co-founders seems to be about perfect.
You can basically change everything in a startup but the market.
You have to find a small market in which you can get a monopoly and then quickly expand.
... you want to be proud of how much you can get done with a small numbers of employees.
In YC's case, the number one cause of early death for startups is cofounder blowups.
Facebook has this famous poster that says move fast and break things. But at the same time they manage to be obsessed with quality.