Hoo boy, this thread.
Now bear in mind that people do have different opinions on this matter and that there will be contentious debate about this particular topic.
That being said...
I personally think it's down to the individual. You yourself know what your intentions are: do you want to play for fun, to improve, to win tournaments, just because you enjoy the game? That will make a huge distinction to how you perceive your main(s).
I use the optional plural because some people have more than one main. And that's fine. You can argue that it defeats the purpose of it being a main, that the other is just a secondary - but really, at the end of the day, it's all arbitrary. You decide how many mains you have and why.
Personally, I believe you can have one or two mains. They are your best characters; your favourites; your go-to guys; the ones you can reliably and comfortably play and expect to get good results with. Beyond that, you can have a few secondaries - I feel like five is the optimal number here - that you keep as subs for when you need to switch out or just want to do something different. The distinction here is not sacrosanct, nor is it empirically defined: it's totally down to your own interpretation and personal commitment. It may be because, like me, you decided your mains based on your favourite characters and the ones that you use the most; it may be because you feel you're not quite as good with your subs as you are with your mains. That's up to you to decide. Again, it's arbitrary. You know yourself what works for you. Play with however many or few mains you like.
For me, I have two mains: Pit and Palutena. I knew from the get-go that they would be my mains, I made a conscious effort to play them and be the best Pit and Palutena player that I can possibly be. Then I have a few secondaries that I love, but either not to the same extent or that I am simply not as skilled with. I play to improve and to have fun with others, so I will generally use my mains if I want to play well and win, and my subs if I want to even out unfavourable matchups or try a new playstyle; sometimes I just play my subs because I want to play them. There's no hard and fast rule that says that a main has to be one character or another. Just bear in mind that you may want to decide on one or two mains if you want to actively master them, and have the others as subs or pockets. Just a thought. It's totally up to you, though, so just enjoy the game, have fun with it, play around and decide what suits you best and why it suits you best, alright?
