Welcome aboard Gomi.
Only one or two well placed punches are needed to knock a guy out cold. Never sacrifice quality for quantity. The key is timing. You need to watch your opponent's pattern and rythm and throw a haymaker at the right moment.
It depends. For someone like Ali, Leonard, or Holmes, you are much better off just rushing them swinging for the fences. But against someone like Foreman, Marciano, or Hagler, pinpointed, accurate shots and quick flurries are better. Unless of course their hurt, then keep throwing in a controled manner.