Depends upon the environment, I'd guess.
In space, where nothing comes easily, it would take a lot.
On Earth, where there is a relatively benign climate, all the O2 one can breathe, food just growing out there, and with control of fire it wouldn't seem to take very many.
Man is a predator, constantly fighting wars, creating enemies where there aren't any as an outlet for those aggressive parts of him. On a spaceship there would be no outlet, and the folks would splinter and destroy themselves relatively rapidly.
Arf