I have one tray for 2 cats. It is the size of a bath tub though! When I got Jaffa and Magpie, I didn't know about the one tray per cat plus one rule, so just stuck one big tray in the corner and they were happy to share for 8 years. When I got Mosi I added another tray, partly so that I could put the litter he was used to in that and take off the door, but they both just used both trays so I got rid of the extra one and they are both happy to share. It does depend on the cats though and it's a good idea to start off with more than one. If they are happy to share then that's great, if not you will have to always have 2 or 3. It also depends on the size of the space. If I lived in a house instead of a small flat, I'd have an extra tray upstairs regardless of whether they were happy to share a tray.