I say it depends on how strong and how often you use them and how similar their styles are. Like I use six, but I consider three my main ones and the other three subs.
You can only "main" one character at a time, but some of your subs can be "upgraded" to mains depending on whom you're focusing on. For instance, I've played 4 different characters in tournament before, but switch between Ayane or Christie based on mood or situation.
I had a lot of characters I used when I was 16-17 but eventually I narrowed it down to the ones I felt strongest with, and I just pick one of the 6 I like depending on how I want to play. Kasumi is usually my go to since I used her longest.