I have two cats, a ferret, and a boyfriend in an 850sqft 1-bedroom apartment. We have plenty of space, as far as I'm concerned. The layout is fantastic, so the litter boxes (which take up more space than the cats) don't bother me. They have an entire hall closet (double doors) for their litter boxes, now that no one is confined to the bedroom. No humans have to step on tracked litter, and the boxes are out of sight (but easily accessible for regular scooping). In the past, I have lived in places that sort of forced placement of the litter boxes in out of the way, inconvenient areas, and it was a pain to stay on top of litter box detail. I don't want them underfoot, but I need to have ready access to them to stay on top of regular (usually twice daily, but always at least once daily) scooping. If i ever build a house, I am including a "litter closet" - it's really one of the major reasons I picked this apartment. As we have three other closets (two walk-ins and a large linen closet), I don't feel like I'm "wasting" closet space. There's room to keep the broom and vacuum in the same closet, and a shelf along the top where we keep laundry items (no in-unit washer/dryer, but the trade-off was worth it).
I think I could easily have several more mellow cats in this space. My lease won't allow it, and my budget certainly won't, but I can see how it could be done.