I too agree with what others have said, that it really all depends on finances, age, time, space, etc.
In my opinion, if you have so many horses that you cannot go out everyday, walk among them, put your hands on them, get down on their level and check them over, and to know each one individually then you have too many. If your horses manes are dreadlocked, because you don't have time to groom them, you have too many. If you give your horses the bare minimum, and you can't provide much else for the number you have, you have too many. If you have overcrowded, overgrazed pastures and no where else to turn them out, you have too many. If you have every stall filled, with horses sharing stalls, horses living in the aisle way, IMO you have to many.
I currently own 2, a mare and a gelding, and both live in a tiny barn out back behind the house. I think I could handle a few a more, but I know logically I can't. While I am 17 and active, I would be afraid I would not have the time for more than 3 horses at this time, should I add another into the group. I run all day between school, my personal life, and taking care of the horses every possible need, I don't have time for much else. In addition to all of this, I am also getting ready to leave for college, training my gelding, working my mare, and applying for jobs. So by the time I factor all of this in, especially if/when I get a job, I fear I would be running 23 of the 24 hours of the day. A third horse, I could do it I think, not much different from the two I already have and have been caring for and managing myself, with help from no one, for the past 4 and a half years. Anymore than 3, there'd be no way. As well, as I have gotten older, and begun to realize there are other things in life than running around with a herd of pretty ponies, there are many other factors in the care of horses. I do not have the room for a 3rd horse. I barely have the room for the 2 I have. I would love an arena, a round pen with good footing, but if I expand my facilities, then I lose pasture, so it's a lose lose scenario. So in light of this, the third horse would need to be boarded out, hopefully at some place very close. But that is another cost, among other horse expenses.
Well, that was very long winded.
I think in the end, it all comes down to the amount of money you can afford to spend on care, including supplies, shavings, feed, vet farrier, trailers, training, shelters, fencing, barns, the amount of time you can give them, your experience level, and age. If you beging to feel overwhelmed, or that you can't spend much time with the horses because you're always feeding and cleaning, then I think you can easily tell when you have too many. Eventually, you will find a happy medium with a herd that's perfect for you, and your situation.
Dan.