Someday, you need to build a business that's difficult to replicate. This is an important part of a good idea.
Great execution towards a terrible idea will get you nowhere.
In YC experience, 2 or 3 co-founders seems to be about perfect.
It's difficult to get large groups of people, to the extreme levels of focus and productivity that you need, for a startup to be successful.
In YC's case, the number one cause of early death for startups is cofounder blowups.
Why now, why is this the perfect time for this particular idea, and to start this particular company?