It depends on the person....and their job/time at home. Some are able to take care of upwards of 50, some just a few.
We currently have....3 minis, 4 big horses(have to tell you guys about the new one...hehehe), a mule and a donkey. Everyone gets fed/watered and their feet and health taken care of on a regular basis. The minis get their pens cleaned daily(horses are out on large dirt pasture, so only when we have a tractor or something). The mule, donkey, and 2 of the 4 horses are pasture ornaments--either too old(donkey and one horse), or mentally messed up(mule, it was the orig. owners), or unbroke and partially lame(1 horse, she has some leg problems that don't allow much work). Everyone else is driven/ridden or will be(ie: too young yet).
We try to be careful about what we take home, as usually, that animal has found a forever home...so we don't want to 'overload' with too many to work with.
Because I know I prefer to do 'one on one' time, and, fairly soon, I am going to be 'on my own', I would rather not have more than the 2 minis that I own(Dan is my dad's horse) and 1-2 big horses...I could have one more mini, but it would have to be a very nice, exceptional CDE horse(perhaps even a pair or tandem mate for one of my current minis). I don't like to have 'potential' sitting and wasting in the pasture, so I want to show/train them....but if there is too many, it doesn't get done.
Edit: I guess I should add after reading billiethekid's post....vet care is a huge concern...if you can't take care of them when they get sick....then you probably have too many. Part of the 'benefits' of going to vet school will, eventually, be that I can provide my own vet care....however, you know what they say about the 'cobbler's children going shoeless!' Pippin, when he had colic surgery, had it for a stupid impaction caused by a sudden change in weather, and thus, less water in his system. Just freak stuff....very expensive freak stuff, I might add.