Well, it's not a matter of "I enjoy playing them" it's a matter of "I split my play time equally between them, more than any other characters."Even from a casual perspective, I still think the word "main" ceases to mean anything when applied so liberally. You don't "main" every character you enjoy playing, and calling every character you play your main is a trap I see a lot of casuals fall into and they usually end up sounding like this:
I enjoy playing pac man, but I don't play him nearly as much as the others, for example.
Again, it looks like there's not a clear consensus over how many mains you can have nor is there a strict definition outside it being the "character(s) you use most" so... -shrugs- not really the biggest issue in the world if someone says they have lots of mains in my opinion.