Dear Ivanhoe,
How many cats can one person own before they’re considered “the crazy cat person?”  How many cats are too many?  Is there a limit?
—Cats Are Too Stinky

Dear Cats,
There are a number of variables that must be considered in determining the absurdity of cat ownership, the absolute number of cats owned being only one of them. Foremost among these are those related to the hygiene and well-being of the cats. For example, the amount of property one owns and the cats’ access to food, water, clean litter boxes and medical care are important. Cats are naturally territorial and keeping too many cats in close proximity can result in cat-inflicted injuries. Cats should also receive an adequate, nutritious diet, but should not be overfed, and should live in a clean environment with access to a clean litter box or other place designated for excretory functions. The reason all this is relevant to your question is that anyone who forces animals to live in artificially hostile, unhealthy and/or unsanitary environments is not only crazy but criminal. Therefore, a person who keeps ten cats on several acres and has the time and resources to maintain their health and cleanliness is less crazy than a person who keeps six in a squalid apartment covered in dander and drenched in urine.
Another set of variables to consider are social in nature.
A person who talks to one’s cats as if they were children, forces them to wear clothing and takes them for walks in a stroller can be perceived by society as pretty crazy even if they own just one cat. The amount of time one devotes to talking about, looking at photos of, taking photos of and blogging about cats can also lead one down the road to crazy-cat-persondom. When a person can only effectively communicate about cats and little else, that person may also be considered crazy, regardless of how many cats they own.
There are some other variables that might make people label a person the “cool cat person” as well. Cat trainers, zoologists and veterinarians who own several well-cared-for cats have the potential to be viewed as interesting people that others might actually want as friends.
Getting back to your question, though, assuming a person keeps clean, healthy felines in a 1,200 square foot home with a small yard, five or six cats that bond well would be borderline and seven or more would be certifiably crazy.