3. 1 or 2 developers maximum (unpaid yet vested in project success), because the Mythical Man-Month
Have you actually read MMM? My recollection (it has been a while since I read it) was that it suggests a core team size of a half dozen to a dozen, though with carefully delineated roles. The Monero core team is actually quite close to this ideal, though perhaps that is not so apparent from the outside. We have also done well to identify important projects that can be done largely independently of the core team (i.e. they have their own core team of manageable size). This is not an accident.
I agree with you on refinement with a clear concrete specification the author of MMM was working for IBM. Whereas I was answering to the innovation stage where there is no specification and everything in the entire holistic iterative design-by-inquiry is open to change. In that case it is difficult to create orthogonal roles and communication overload or miscommunication or disagreement gets in the way.
P.S. I edited my prior post in a few areas to make it more fair and balanced, e.g. the #1 answer and the link on the word "theoretical" to emphasize it is not a widely accepted fact.