Well respectively I would say using all characters would give you more of an advantage, but if you're going to play competitively I would use 3 or 4 characters at the most. If you use too many at once your just gonna be okay with the ones use ;). 

I cannot, for the life of me, play anyone other than Hazama in Blazblue. Even running through the trials for characters, nothing ever sticks. Hazama's did, with little effort, and I found myself actually getting better and better rather quickly (I never earnestly played fighting games until Continuum Shift). It might be a good thing to have basic handles over other characters (I don't, I just learn by fighting against them =P), but unless you WANT to be good with specific characters I don't really see much point in going into detail with other characters.

I suppose MvC is sort of an outlier example, since you're sort of required to have a degree of knowledge about multiple characters. You know, to build a team and what not.

But for other fighting games, I'd say it's practical--if not entirely necessary. I mean, obviously there's a certain ceiling to how much one person can internalize about multiple fighters, but it's good to have a couple different tools in your toolbag, if you follow.

In high level competitive play, one would have a working understanding of the whole cast, ideally. You're going to need to know the various weakpoints of your many varied opponents, and you're going to need to understand what characters exploit those weakpoints most effectively--or rather, which of the characters you're intimately familiar with can function in a way that best takes advantage of the opposing characters' flaws.

Ultimately, fighting games will always be more about the metagame than the game itself. You're better off getting practice adapting to situations or learning how to read animations (moreover, expect animations in said situations) than you are just memorizing combo strings. You might be able to rattle off a 14-hit, 38-percent damage combo with Reptile in practice mode, but if you don't understand the best ways to set that up against a live opponent--particularly a live opponent with thoughtful defensive strategies--then you'll be boned before the match starts, in which case it wouldn't matter if you knew the moveset of the entire cast up and down and backwards.

I main Ryu and Fei Long in SSFIV. Change them up pretty much every match. Or whoever's better for the character my opponent is likely to pick.

i guess around 3 characters would be good. i don't really play fighting games a lot. i had bloody roar: primal fury for gamecube. i only used one character in that game. the guy would turn into a alien type thing with knifes everywhere. he was cool.

I prefer being a master of one than a Jack of all trades. So for fighting games that are not team oriented like MvC, I think it's more practical to main only one character if you want to be competitive. Of course, you need to play other characters in order to understand their strengths and weaknesses.

You should at least use each character once or twice , just to know their move sets and or type of powers , weakness and strengths , but to main all is just silly IMO , also there are characters that I would never ever use outside of what I mentioned , or to give some one a chance at me , altho to be honest I am a medium low grade player ... is just that my immediate non online opponents are not very good at it :\ .... but I never ascribe to the "oh this is god tier character you can only use this guy" crap , a good player knows his main and wins with it. I personally try to main at least 2 , to have variety in play styles , 3 is ok , 4 may be stretching it a bit much ... more than 5 is too much.

Play all to get to know their skills/moves then main the one you're comfortable with.

I play Chun, Cody, Sakura, Dan, and Ibuki. And I use to be very good with Blanka and OK with Dhalsim in the original SF4.

UMvC, KOFXIII and SFxT will require you to master multiple characters, preferably ones that play well together.

In SSF4 I play Dudley and Akuma. In Mortal Kombat Scorpion and Sub Zero. So I guess twos a good number.

I main Yuri and Gouken in SSFIV so there.

Um, just have fun. Your ranking means absolutely nothing to anyone in the real world. So have fun.