What they said.
A main is your go-to character. You will generally know this character in and out.
You will know all of their moves, what moves link into which other moves, how fast their moves come out, what their strengths and weaknesses are, who they match up well against, who they do not match up well against, etc.
After that point, you will have alts. These are a few characters you can whip out depending on the match-up or just to spice things up. You might know them as well as your main, and you might not. Depends on you really.
I don’t see a problem with playing 17 different characters if you’re having fun with them, but at some point you will probably like to start narrowing down your playing field to a handful of fighters and start figuring out who really fits you the best and learn them inside and out.
Once you understand the core mechanics behind the game at an intimate level and have one character that you know inside and out, you will be able to pick up and master other characters at a much faster rate (at least, in my experience).
The problem with switching between 17 different characters is you will never be able to master one, you’ll always just be decent with 17. It’s kind of like being a jack of all trades. Yeah, you’re probably pretty sweet at a lot of jobs, but you’ll never be as good at a job as a specialist who does one thing and does it better than anyone else.
Same thing. You can probably pull out some sweet moves with all of your characters but when it comes down to the wire, unless you know the character you’re playing at a higher level, you’ll probably choke in contrast to someone who just plays that one character.