I own 7 cat but Sofa and Freddy live with my parents in Gloucester as Sofa is pretty much blind and she knows everywhere and they live inthe country in the middle of nowhere and so Sofa is not at threat of cars, etc. Freddy became Sofa's closest cat and he is very settled and so when I moved off my parents land sounds more posh than it is) it was kinder to leave them with my parents for Sofas good as she is an old soul and the unhealve would have been awful for her. I still pay their insurance though. Sofa is on a lot of meds also which cost a lot.
We have 5 living with my husband and I, Poppy is the newest and came to us when we moved into this house because the new house was big enough to accomodate 5. Our house is a 3 story 3 bedroom, 2 front room house so its pretty damn large and they all have places they can escape form each other when they want. All 5 are insured also.
Foodwise they cost a lot, especially due to vet diets, and the insurance for 7 hits around the £100 mark each month but obviously ou have to pay out firstly for any veterinary stuff.
I think how many is too many when they start fighting, aggressive behaviour comes out plus you start getting toileting problems - as someones already said. Cats will soon tell you when you have 1 too many. The size of the environment they live in is important too and yes overall costs for everything - it can be bloody damn expensive but me and my husband make sure the cats are sorted first then comes us
My cats are my world, my husband knows that.
I would say with just food and insurance a month you are looking at around £200 minimum and thats without any extras or possible vets costs. Flea treatment and wormers for the 7 ever 3 months is easily £100. It adds up.