Your statement is not necessarily true. Sometimes you can cheat a little by adding more fish and upping the filtration, but that's not usually the best way to keep a tank. African cichlids in a tank would be an example of when this is done on a regular basis because overstocking keeps the aggression down in these kinds of tanks.
You would normally want to run ten times your tank volume through your filter in an hour. For instance if you have a ten gallon tank you would want your flter to be rated 100gph. so when you buy a filter this is what you want to look for.
It won't help you overstock your tank but will help keep the fish you have in cleaner water
You really need to research what kind of cichlids you want to put in your tank. It's kind of hard to say "just get 20 of any kind and chuck them in ". also research what their adult sizes are going to be, that's always a consideration.