The million dollar question, how many cats should I have? There is no magic number when it comes to how many cats, it is reliant on many factors such as the personality of the cats, your home situation, how much space you have, how much time you have. But let's start with the basics.
Your home situation:
Firstly, you need to look at your home situation. Do you work? Will the cat(s) be alone for extended periods of time? Do you live in a small apartment or have a large home?
Cats are known for their independent nature, but if you live in a home where everybody is out for a large portion of the day, then it is a good idea to have two cats, so they can keep each other company. Again, this isn't a hard and fast rule, there are some cats who are perfectly happy to be on their own for large amounts of time. But in general, it is recommended that two cats are better than one in this situation.
How much time do you have? Cats are extremely easy pets to care for but the still require a little effort. Litter box maintenance, feeding, grooming, attention all play a factor in this decision. What about you, are you physically able to properly care for a number of cats? Financially? Can you afford food and vet care for them all? This is in no way meant to be a judgement on anybody wanting cats, but finances certainly play a role when it comes to our responsibility towards cats and how many cats we should have.
In Australia, many councils are now placing restrictions on the number of cats a household can have. Before you adopt a cat, check with your council as to the numbers permitted in your area.
So, how many cats is too many?
Again, there is no clear answer. I have known of multiple cat households where all cats were happy (we used to have five cats who all got along fantastically). I do believe that you should be able to provide your cats with adequate space to be able to have time out should they require. So, having 4 cats in a small apartment is not ideal, but 4 cats in a large house, with multiple rooms, might not be so bad.
The more cats you have, the greater the chance of behavioural issues developing such as cat to cat aggression, spraying and going to the toilet outside the litter tray. One or two cats, you really should see no issue, but as each cat is added, the chance of problems occurring increases. Not always, but the odds are growing.
I am very aware of assessing the situation on a case by case basis. Looking at the current cats, their personality, cat to cat dynamics and deciding if I feel they would accept a new cat. We currently have one cat, he is quite happy to be on his own (I work from home), when we did have another cat (who sadly died in 2012), he would constantly harass her. He wasn't especially aggressive, but he made it known that he didn't like having her around. He is an alpha cat and we have decided that for now, he is better off on his own (he has a lazy Chocolate Labrador to keep him company). I do not think he would readily accept another male, especially an alpha one.
So to summarise, my personal view is that two cats are an ideal number. You can certainly have more cats than that, but before you do, look at your current situation, how the cats are, do you feel they would accept a newcomer to the house. Introduce cats slowly, and see how they cope. If they are fine, then that's great. If behavioural problems start occurring, maybe it's time to put a cap on more cats.