As past topics have demonstrated, there's not a whole lot of agreement on what a main entails, so you're bound to get different answers here.
The way I see it, a main generally refers to the single character a player is most effective with and/or uses most frequently. By this definition, you can only have one main. That doesn't mean you aren't good with other characters; skilled pros spend hundreds of hours training as multiple characters, and some pros become so good that they can win major tournaments with several characters. Just being good with a character doesn't make them your main, though. Your main is the one character you feel most confident with in the greatest number of situations. I main Ike. Online I typically play Ike, Shulk, and Robin pretty evenly, with some WFT mixed in, and I like to think I'm decent with all 4 characters. But when my friends and I play a serious match for bragging rights, Ike is usually my go-to guy. He's my main, the others are secondaries. Some people act like calling a character a secondary is some kind of insult, but there's really no difference between having 1 main and 1 secondary and having 2 supposed 'mains'. Being 2nd out of 51 characters is not bad.
If you do somehow manage to play more than 1 character completely equally, with no favoritism in play time or difference in skill level, then it's not clear to me what those characters are. The strictest definition of a main suggests that you simply haven't chosen a main yet, since you have not identified the character you rely on more, but I'm willing to grant that you essentially have dual mains, for all intents and purposes. Still, this is nigh impossible to do with more than ~2 characters, since beyond that it's fairly inevitable you will perform ever so slightly better with one character than another.
For you situation, Marthmario, it looks to me like you are operating under a looser definition of a main. In terms of just characters that you learn the moves and strategies of, you could probably get away with a dozen or so, given enough time and dedication. But by the more strict competitive definition of a main, it sounds like you don't have a main at all. That's not a problem, as outside of competition there's no need to have a dedicated main. But a main really should be a single character, ignoring rare exceptions.