Let's see. . .
Litterboxes:
I have 3 rooms set up for the kitties, with baby gates/cat doors to keep the dogs out. The "porch" (not sure what else to call it-it's a proper room that used to be the front porch, but is now enclosed, heated/cooled, and insulated), the spare bedroom, and the basement (half of the basement is the laundry room--no cats allowed--and the other half is one big room). There are 6 litterboxes in each cat room (lined up against the walls), plus 2 in Silka's bedroom (she only enjoys the company of 2 other cats, and usually one of them is in with her, they switch off), plus one in the spare bathroom, because that's Mooch's safe haven (the other cats pick on him so he spends a lot of time in there). I use Tidy Cats and Special Kitty clumping, and I dump/wash/refill with new litter one litterbox a week.
I have found that food has the biggest effect on litterbox odor. If I go with the cheaper foods, I get stunk out of the house! So I stick with mid-grade foods (Chicken Soup, 4Health, etc.). They don't cost much more than the cheaper foods, all things considered. And I get a discount at the feed store for being a good customer

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Food and water:
There is one pan down for dry (a large stainless steel "puppy pan" with a raised middle) in the porch room. Also a large ceramic water bowl in there. There's a small glass bowl next to the kitchen sink that they all prefer, I fill it almost every time I go by the sink, LOL. There is also a large plastic water bowl on the "back porch" (same thing as the front porch--now enclosed) for the dogs, and the cats can get to it when the dogs aren't closed back there (when I'm not home). I put 2 large plates of canned food down every night.
Guests:
I don't really like having guests. It's more fun to go out with friends instead of having them over. When people come over, they do get mobbed, but not by all 20! 5 of them are feral and several are quite shy and just hide. But if you don't want to get cat-mobbed, don't come to the cats' house! LOL. If I wanted to have people over without cats bugging us, I'd shut them in the basement.
Cat hair:
Not a problem? IDK, it just isn't an issue for me. Regular vacuuming and a washable slipcover on the couch takes care of everything. Maybe it's the food, but really the cats don't shed much. The dogs, on the other hand. . .
Vet bills:
So far so good. Other than spaying/neutering and kitten shots, most have never had to see the vet. If they need vet care, they get it, but I won't go to heroic measures to save a cat who likely won't have much quality of life afterward anyway (but that would be the same whether I had 2 or 20). They will never be allowed to suffer. Plus my vet is nice and will let me get antibiotics, de-wormer, etc. without an office visit. All I have to do is talk to him and describe the symptoms, and he'll tell me whether the cat needs to come in or what I can do at home.
Attention:
I don't think they lack for attention. Like I said, some are feral and don't want my attention anyway, but those that do want attention are not shy about demanding their due!
I am not a neat freak or a control freak, so if things get a bit chaotic, I'm fine with that. If someone had a problem with chaos, I think it would be terribly stressful for them to have so many cats.
I really do not feel overwhelmed by the cats. I do sometimes feel overwhelmed by the dogs, and after they're gone I will only have one dog at a time. I like having lots of cats around, although I probably wouldn't choose to have 20. Maybe 10. But they need homes, and I can provide that home, so why wouldn't I?