I think, and feel free to disagree, that this discussion is likely not going to lead anywhere - here's my suggestion, and I think most of the folks here would agree.
Get your tank up, running, and cycled.
Add one fish and sit on it. Wait. Check your levels constantly and see how it goes. Personally for a 20 I wouldn't do more than 4 small fish, but that's captain understock talking here. Once you know that the tank is cycled and has enough bacteria to support that fish, add another one. Repeat. I would tend to agree that 5 is the absolute max I would suggest.
I've seen people pack a butt-ton of fish into a tank before too, but just because the stuff is alive doesn't mean its healthy or happy, lets keep that in mind.
firefish, ocellaris clown fish
caulastrea, green star polyps, palythoa, zoas