I think that having changed from a 1 cat to a 2 cat home, it's probably most important to have that 'one for luck' box if you have just 2 or 3 cats. The reason for that is to prevent 'guarding', if one cat is territorial it is likely to want to keep one area as its marker, ie will want 1 box for itself. They can prevent a more submissive cat from using that one box just by sitting in view of it - us humans probably wouldn't even notice the underlying tension playing out in front of us until submissive cat can't hold it in any longer and pees/poos on the bed to alert us to his plight!
If you have 2 cats and 3 boxes, then the more dominant cat cannot guard all 3 boxes simultaneously, so submissive cat can go in peace. If you have 8 cats and 4 boxes, then any 1 particularly dominant individual would still not be able to guard 4 boxes in order to demonstrate his territorial superiority over his more submissive housemates, and there is a more complex pecking order going on, so probably 1 dominant cat is not going to focus all his energy on trying to prevent every other cat in the house from using all of 'his' boxes.
Cleanliness is a slightly different issue, obviously it's going to be problematic keeping 1 box clean enough for 8 cats to feel comfortable and happy to use, even if you're scooping all day there's probably a queue forming. So I would say use as many as you have room for and can keep clean, if you use too few your cats will likely tell you in due course! Each individual cat is different (Radar is not at all fussy about which litterbox he uses for what, whereas Sonic is picky about litter and privacy), and the dynamics in each multicat house are different - so I think the 1 per cat +1 makes good sense for 2 to 3 cats, but beyond that a lot of it will come down to practicalities of your household and the dynamics between the cats.