People will have one account for each character they have. There would be very few people who are playing 20 characters at the same time, but over time people change the characters they play (and some people will pick up characters more often than others). And sometimes you might even have more than one account for the same character, because you're playing them at different ages or something.

I actually have about 20ish accounts, including all of my characters plus my personal journal, graphics journal, etc etc. But I'd hazard a guess that I'm at the mid to lower end of the spectrum in terms of RPers. They are definitely people who'd have way more, and it's just because they've played more characters.