Read "The Mythical Man-Month".

Basically, a team of 5-8 motivated developers can create high quality, medium complexity software extremely fast.
But if the project is just a little too complex for one team of devs and you need more people, then you'll need a lot more people. And a lot more time.

Cause the more people you add to the project, the more overhead you have. Suddenly you need to pull devs off coding to bring new hires up to speed. You need to write documentation on coding style guidelines, hold meetings, maintain your infrastructure, negotiate with hardware suppliers, have someone fix the server room's door locks, schedule job interviews, etc. etc.