Ulk made a great resume, but there is another aspect that wasn't covered... In tournament, let's say you have two mains, you have to decide who you playing first game. I know this can be character based, taking into consideration who has the best matchup against the character you're facing but that isn't the only deciding factor. You main Mario and Falcon, let's consider you're going to face someone that plays bowser jr. (just as an example), as although that's considered one of (if not the best) Mario matchup, but you've seen the same player destroy three mario players in a row in pools. Do you then consider not playing Mario ? (i'm not assuming you do but that's a possibility when you dual main) Do you then go falcon ? It's still a favorable matchup, but isn't it easier to kill Falcon than Mario ? Is it going to be a good choice ? Can you outplay the Bowser Jr. Player with your Falcon?
If you go Mario and lose game 1, do you stick to Mario ? Do you change characters ? If so, why ?
What i'm showing here is that maining more than a character, although can be a great thing for checking bad matchups, can become a issue if you're not feeling confident that day or lose a game against the player, or is facing someone who is way higher than you on power ranking or someone you often lose to. Whatever gets you to the "i'm not sure who to play" mindset. Momochi, a Street Fighter player, said once in a interview that a player should stick to a character and learn every weapon that character has to deal with every matchup. Train every matchup the hardest they can and build a plan, sticking to that plan no matter what and if the plan is the right one and your execution is close to perfection you have a chance to win even a 7-3 matchup. This isn't only motivational theory, he proved himself several times by winning several bad matchups by knowing the few things he could do. So yes, you can have only one main if you put enough time into it so you can avoid doing the bad stuff against character X and have a plan on what you can do against it. Playing more deffensively, not giving frames for them to punish and punishing whatever mistake they make, not overextending, trying to be always on % advantage and you can win the match.
When mindset comes into play added with the time/effort you have to put in two character that Ulk posted right before me make the question of "should i main more than one character" even harder. If you feel confident about mastering more than one character and having the option of a switch doesn't add weight to decisions you have to make before game 1 and mid match, you can have as many mains as you can, but I definitely always go for stick to one or two characters and choose them by coverage, having the secondary just to cover specific matchups you're not comfortable doing with your main.
as my main. And 
