It's a bit subjective, since it differs based on the person, the time it takes to learn characters is different for everyone, but I've always seen 4-5 characters as the general point where it's kinda crossing the line. Three characters is enough to cover basically every relevant match-up, and while it's feasible to put in enough time to both master 3 characters and keep them at a competitively viable level, I'm not really sure how realistic it is to go past that point. Consider this, time-wise to master 5 characters, if you put equal time into all your characters, you'd be splitting your time up in a way that limits your character as having 1/5th the amount of time it could receive compared to someone solo-maining that character. That number only gets worse the more characters you have, since the fraction would just get smaller and smaller. If you can learn faster than most people you could probably handle it, but it's still a limiting factor since you'd still be putting in less time to your individual characters than you could be.
However, that's really only something to be worried about if your into the competitive aspect of the game, if you don't care about that and just want to have fun, you can really main however much characters you want, 2 characters, 16 characters, 53 characters, whatever floats your boat.
Edit: Grammar