Well, in one of my Pokémon fics, I have four main characters, one of whom is the main antagonist, and maybe four or five lesser characters who still manage to play pretty big roles. Count: probably no more than six at a given time.
In my main Pokémon fic series, however:
Part 1 - four main protagonists, one or two main "bad guys" who probably won't get more than a chapter or two to themselves, and one lesser character. Count: no more than five major players at a given time.
Part 2 - three constantly appearing protagonists, at least two major antagonists, and probably five "lessers," one or two of whom will end up dead by the end. Count: no more than six or seven at a time.
Part 3 - Six major protagonists, one major antagonist, but no lessers. Count: seven at once throughout almost the entire thing, not including short appearances by various trained and other pokémon, of which there will be many.
Part 4 - All of the major protagonists who are able to return do so, so seven of those, plus the big final "bad guy" of the series. Count: eight.
I'm also part of a fantasy roleplay in which I currently have the reins of five characters, but by the end that'll be upped to, like, seven or eight with the two final major bad guys. As far as I know, that's my limit, but...
I've also been planning out a Digimon fanfic. I haven't planned out every single character just yet, but I'd say that by the end, with every human character and a Digimon partner for most of them, I'm gonna end up with upwards of fifteen different characters. Truly, that will be a test, and I highly doubt I'd be able to go far if at all beyond that.
I'd say that "too many" only occurs when the author is unable to give a decent amount of scene time to each one and generally isn't able to keep track of everyone. This actually happened to a friend of mine in the above mentioned roleplay; he had probably fifteen characters at one point, and he decided to kill off (or otherwise get rid of) everyone who wasn't all that important--quite sloppily, I might add--and left himself with four characters, another of whom got killed off a short while later. And he's told me that he plans on getting rid of yet another one soon after the thing picks up again; between college, jobs, and friends, he just isn't able to keep track of even three characters.