I usually took 18 credits while working 60+ hour weeks, the majority of those hours being 3rd shift. It's doable, unhealthy, I hated it, and my grades weren't as great as they could have been--but I really didn't have a choice if I wanted to keep a roof over my head and eat. It's not normal, so I'm confused at your confusion. So many of my colleagues, pre-professionals alike, constantly informed me that I was crazy and there was no way they'd be able to do it (let alone take 18 credit hours consistently).
Everyone handles their workloads differently and the intensity of courses may vary significantly by school. I wish I didn't need to take that many credit hours at once, I think I would have enjoyed my undergrad more (much like those did who took 14-16 credits hours, which is the real norm). I would never advise any student to take that many credit hours semester after semester if it wasn't a necessity.
So you took 20 credit hour semesters during your undergrad---are you expecting a medal? I'm not sure what you are looking to gain from this thread, and you're more likely an attention seeker than a "confused" poster. Perhaps that person you are referring to over-ambitiously took 18 credits worth of ridiculously challenging courses--regardless, that many credits can be brutal for anyone--except for you, of course.