Let's say I want to get a 20 gallon, I should go for a 20 long rather than a 20 high, right? Like I should be able to have more fish in a 20 long because of the larger footprint?

Theoretically, yes, but then you have to remember that they hold the same amount of water. It's just that, say you have danios - which are small fast moving fish - They'd prefer a 20 long because the horizontal space is more useful to them than vertical space is. But you couldn't have loads more danios in a Long for water quality reasons.