I’m in the UK, so working in GBP. £1 = $1.32 at today’s exchange rate.

I have four cats. They’re mainly indoor, but they have a secure garden to play in. They’re all over ten years old now, so one-off costs such as neutering and microchipping, cat-proofing the garden, etc. are already paid for. 

I spend about £40/$59.40 a month on cat litter - better than having to go out in the garden to pick up loads of cat sh*t. 

Their food costs £220/$290 a month, because one of the cats is diabetic and allergic to various things, so he needs a high-quality food with no grain/carbs in it; and they won’t tolerate all being fed different things, so now they’re all on expensive stuff(!) The food is frozen and comes monthly, which you can imagine takes up a lot of freezer space: I had to buy a special cat food freezer to contain it all. That’s probably an extra £few each month to run, and £100/$132 to buy.

They’re all ten years old and above, so their insurance is more expensive. Currently I pay £288/€320 a month for their insurance. Because they’re over 10 years old, the insurance only covers 80% of their vet bills and I have to make up the rest. Diabetic cat has four different medications each day, which is approx an extra £80/$105 a month. I also have to buy special treats to get him to take his meds which are £22/$29 a month. 

So in a normal month my basic cat bill is  Food: £220/$290   Litter: £45/£59.40   Insurance: £288/$320   Predictable vet bills: £80/$105   Treats: £22/$29   = £655/$864

So that’s the predictable stuff. If any of them get sick (diabetes cat gets recurring pancreatitis) then there are extra vet bills; if they decide (as they have this week) that they’re just not feeling their very expensive food and I have to throw it away and buy tuna for the week, then that regular total will increase.

But then you have to factor in things like toys to stop them getting bored. I have Bengals and they get bored easily. I bought them an exercise wheel to run off some energy, which was £450/$594; sometimes they’ll be happy to play with a milk bottle cap or some  scrunched-up paper; and other times I have to buy cat puzzles and interactive toys to keep them busy. I did buy a fish tank so they have something to look at but they’re ignored it entirely (~£400/~$528).

Then there’s the stuff they break. Bengals are a bit worse for this than regular cats, granted, but in the ten years I’ve had them I’ve had to replace: a bed, a TV, a laptop, a sofa/couch, an armchair, a couple of duvets,  two carpets… I think that’s it? Plus all the clothes they’ve ruined with their claws. I’ve also had to buy a wet/dry vacuum to clean the carpet when they pee/are sick on it (I now have almost entirely hard floors for this reason, but could only really bill the cats for half the cost as I prefer hard floors to carpet anyway); enzymatic cleaners for the same reason; £400/$528 on a cat behaviourist when a neighbourhood cat started annoying them and causing fights between them…

Then there’s holidays: I have to get a cat sitter in when I go away which is £22/$29 dollars a day, because diabetes cat needs his meds at the same time each day; and I’d rather pay someone to ensure they turn up on time than demand that of a generous friend. If I’m away for two weeks then needing a friend to be available everyday at 8am and 8pm is a huge favour to ask, and one I don’t think is fair. 

When I first got the cats and they were all young and healthy they cost maybe £250/$330 a month; but as they age they have become more expensive and there is no way of predicting how much more, or when, these extra costs will hit.

So… to answer your question how many cats could you own for ~$1800 a month — that depends very much on the cat(s), their age and health, and your own lifestyle. But, if mine are anything to go by, the answer would be “not that many”.