For me it's less of an issue of too many characters than it is the routes becoming too short if there are too many. If there's a game with 10 LIs and a game with 5 LIs and they're of equal length, then unless there's some masterpiece-level writing in the former I'm going to prefer the one with fewer LIs as it's more likely they will have more satisfactory, in-depth routes.
I can somewhat get the appeal of more LIs if the game is solely focused on fluffy romance and doesn't have any huge plot going on, but even then those games aren't to my taste anyways.
So, for the average-length otome game, I'm most comfortable with 5-6. (4 is also acceptable on the shorter end but generally I prefer at least 5.) For a longer length one pushing it to ~8 is fine, though IMO anything above that does get a bit tedious.