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.