I almost agree with point brought up that a main is only the character one uses most. By definition, the word most that the character will receive the most usage at the time and unless someone is literally using characters equally, there will be one main. We don't know, however, if we are or are not using multiple characters equally or not, so we just say we have multiple mains to make it easier even though technically, there probably is a character a player is using more than the others. What the OP wants to know is how many characters he should focus on, whether it be completely equal focus or not, in order to be successful competitively. This depends on a multitude of factors.
It depends on the meta of the game being played. In a game like Virtua Fighter 5, the game extremely balanced (the tier in the game are only A,B,and C tier), so a player can realistically succeed with one character. Focusing on one character in Virtua Fighter is even encouraged because of the uniqueness of the characters and the difficulty of just learning one character. In games that are less balanced, then you need to judge whether your character/s need a back-up. At the higher tiers, the characters can stand alone in the competitive environment. On the other hand, lower tier characters will most likely require secondaries in order to succeed. The number of secondaries need in each tier of a game is also based on a character's match-up polarity. If the low tier in a game have match-up spreads that consist of match-ups all equal to or better say 4:6, then they actually can be used alone because it's only a slight (but consistent) disadvantage. However, if a character has polarizing match ups, whether or not they are high tier, then secondaries may be necessary.
Already previously noted, complexity of the characters at question is a factor. If the secondary required to help even up a character's match-up spread has a high barrier to play, such as SF4's C. Viper or Guilty Gear's I-No, then maybe it's in your best interest to just struggle with the bad match-ups. Ideally, a player should always pick the best character for every match-up regardless of technical barriers. But less be real, people and situations are less than ideal.
Last is just personal preference. Playing characters that resonate with you, be it through a character's playstyle, look, history, or other factor, can just be more rewarding. Fighting yourself just to play characters you don't like can potentially negate the competitive benefits the character should give you. It's like working a job you hate just for the pay. Sure, you get good money. But, you struggle to truly grow because you lack the will to. The same thing can be said on not playing characters because you are told you're focusing on too many characters or a certain character is bad.
In Smash 4, I feel that ideally a player should focus on 2-3 characters, but it is feasible to play only 1 or more than three. In it's current state, the balance is pretty good. The top character don't really have any terrible match-ups so they can stand alone. On the bottom end, there actually aren't too many characters that face a vast amount of polarizing match-ups, so they may only need one secondary to cover a few match-ups. Characters like Little Mac, Mega Man, Bowser, and Olimar are example of exceptions. Smash 4 is easy enough to play, so it shouldn't take too much effort to learn and focus on most characters. Peach and Rosalena and Chiko are example of pretty difficult characters to play though. On a grander scale, it may be best for you to stick to only a few characters if you are new or inexperienced with competitive Smash. Fundamentals are will get you farther than character selection and learning them should be the main focus of your play. Ultimately though, your personal option is the final choice. Wanna play Link because you like The Legend of Zelda? Go for it. Play Kirby because he is cute if you want. Don't let competitive idealism take away from your play experience.
I hope that helped. Good luck and have fun.
, 
