The general rule is to have one litter tray per cat plus one extra so 7 cats would mean 8 litter trays.

However it all depends on the cats and how willing they are to share trays.

I have 5 cats and 2 foster kittens at the moment and and I have 4 litter trays which they are quite happy about it.

You do need one on each floor to make them easilly accesable.

I would try with 4 trays and see how they get on with that, if you find any not using the tray then try adding more until the problem is solved.

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