I kinda have the same problem, I too am playing more characters than necessary and it can kinda hurt my skill level too. I shouldn't really go beyond 4 or 5 mains, but I can't help it:
My main and my 2ndary:
Sidekicks, not my go-tos, but would sometimes use in a tournament:
/
I also want to plant Ness in my main train too, since I love him in the other Smash games and has always been one of my favorites, but I think it is getting out of hand for me at least. And there's ones I play exclusively for fun. I don't really take them
too seriously and would never actually use any of them in a tournament, but I still practice them to an extent and take away time to practice my real mains:
Yeah, I dunno about you, but I've definitely gone too far with this. I can't help but feel that so many of these guys are just soooooo fun to play, but having only 1 or 2 characters is generally considered the best options since you have more time to focus with them. Someone just might tell you this:
Why be decent with many characters when you can be GREAT with one?
However, at the last two tournaments I attended, I still got pretty far with 5th place out of 35 people the first time, and 7th out of 42 the 2nd; I'm surprised I've gone this far when I don't even have a lot of competitive experience with the other Smash games yet, Smash 4's time is basically when I started entering the ring with it, Melee, and Project M. So considering how far I've gotten in my experience in my 2 recent times, I don't think playing multiple characters has hurt me TOO much, and the same could possibly be said for you to as long as you don't divide your mains too evenly. I've been told I'm good with Greninja, Sonic, and Duck Hunt; while being Decent but not great with most of my sidekicks except Pit, Palutena (I'm getting better with her though), and Robin.
I think I've found a solution for this kind of thing myself: Simply Limit the Amount of characters I would actually use in a tournament, and separate them from ones I plan to bring to tournaments, but am not yet ready.
Tournament Ready
Greninja
Sonic
Duck Hunt
Megaman
Pac-Man
Working on (from greatest to least in terms of readiness), don't use them now
Shulk
Marth/Lucina
Palutena
Robin
Pit
Ness
MAYBE Will practice for tournaments in the future
Pikachu
Rosalina (my little cousin plays her though)
Bowser Jr
Assuming your intent is tournament success, try doing what I do and see if it works for you. But if you never plan on entering tournies, maybe it won't really matter that much. Just only focus on 3 or 4 at a time at most, and then wait for a month or two and then try another handful. And remember that you can only really have one main and one secondary, the rest are mostly pockets.