I have 7 cats and once had 10 trays

but only 5 ever got used. So they have 5 large ones all covered and in various locations. They seem to like this layout. If I have any more cats I would dig another tray out and see how it goes.
I scoop as soon as I see a solid in the tray and completely change the litter every 2/3 days and give them a proper clean etc.... sometime more often.