I feel like you can have multiple "mains", but the use of the word sticks better when you have just one character as your "main"
For example, in Project M, my "main" is Snake, who is without a doubt my best character.
I'll alternate between Link, Roy, and Ganon, and those would be my secondaries, rather than my "other mains"
It's really just based on your interpretation of the word. For Smash 4, my "main" is ROB, but i'll mess around with other characters.
So either your best character is your only main, and all other's are secondaries, or multiple characters are your mains, and the rest are just characters you pick out of boredom.