At YC we have this public phrase, and it's relentlessly resourceful.
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.
You need conviction in your own beliefs, and the willingness to ignore others naysaying.
In YC experience, 2 or 3 co-founders seems to be about perfect.
... if you talk to say any of the first 40 or 50 employees, they all feel like they were a part of the founding of the company.
Remember that the idea will expand, and become more ambitious as you go.