An interesting (and applicable to my life) Ask MetaFilter question: How many cats is too many.
Many responders agreed with this reply:
One or two is reasonable. Three and you're a little odd. Four is borderline weird-o. Five is certifiable craziness.
3-4 cat lover.
5-6 Excessive. Suspicion that the owner may have abandonment issues :)
7+ Warning bells. Person at risk of a lonely death with only feline companions who have overrun the house, and will feed on the corpse when the catfood runs out.
1 less than enough to smell.
I agree. My roommates and I have four cats between us (though in a big house) and, thank jeebus, our house does not smell.
No one mentioned anything about cat hair. I would say 'only so many cats that make you vacuum no more than once a week.'