Some folks get really hung up on hobby 'rules' that are more myths than laws.
My water is neither hard nor soft with a pH of 7.6....In my 60g I have Angels, neons, cories, and swordtails (and have for years) and they all do just fine! (I also bred, raised, and sold 50+ Mollies).
In the wild, some fish are found in waters either hard or soft so hobbyists think that's the only water they can be in. In reality, many of the fish we see in the hobby are bred and raised in tanks and/or ponds with various water chemistries, often nowhere near the same as if they were found in, or caught from the wild.
I'm no expert, and others will disagree, but I have been in the hobby for 50+ years and
In my opinion, based on my experience, most fish can do nicely in water chemistries that differ from their 'wild' cousins. It's far more important to have and maintain clean, fresh water that's as pollution free as is possible. You don't do this with filters, but rather fast growing plants and routine partial water changes.
As to your question
@Haywire, there really is no precise answer. (And I cringe at the silly 1" per gallon 'rule'). In a 47g tank, anything over say about 15-20 small fish could be considered crowded. But crowded is a relative term. If the water quality remains high due to increased maintenance, more fish can live well in the same space. We run into trouble when we slack off even with fewer fish in polluted water. In reality, the larger the tank and the smaller the bio-load tends to produce the best results as water quality remains better, longer.