I have a cat litter box question?

I help with a rescue and have 6 cats currently (All spayed and neutored). All the cats have some kind of special needs. I love every one of them dearly they are my four-legged kids LOL. Here is the question what automatic litter box would you say is best for this many cats and depending on what the needs are can have two or three more so let me know please. The six cats will never be able to be adopted so I will keep them unless a proper home is found and where I live that is very slim it would ever happen. The chance of me giving these animals up are slim to nil though because I have had all six of them over a year. LOL Thank you ahead of time for answering this, I appreciate it very much. A side note I have a huge house plus the cats have their own room to play and romp around. They have 4 or 5 huge cat trees so FYI these cats are taking very very very good care of. DO NOT BASH YOU WILL BE DELETED!!!!!! TY and God Bless