I have seen a few suggestions in answers to other questions about it. But when I look I can never find it.
If I have multiple cats in my home, how many litter boxes should I have?
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Rule of thumb is n + 1 litter boxes for n cats where n is greater than 1, this ensures a plentiful supply of this particular resource - with cats behaving more as individuals than a co-operative unit (such as a pack) they are more likely to accept sharing territory with another cat if there is no competition for key resources (toilet area, food, water etc)
It's by no means a hard and fast rule - my two get on just fine with two boxes between them but particularly during the introduction phase ensuring no or minimal resource competition is highly recommended.
The popular answer is "One box per cat, plus one extra", but I think it's a bit more complex (or a bit more flexible, depending on how you look at it).
You may need more than "cats + 1" boxes if:
You can probably get away with less than "cats + 1" boxes if your cats are happy to share, get along well, don't resource guard, are in a smaller-but-also-not-too-cramped space, and you clean the boxes frequently (daily or more often). If you prefer to use very large boxes, that could also possibly reduce the number needed, as long as your cats don't mind sharing.
I have always used 2 boxes for my two cats with good success, and they can share one box for short periods without seeming stressed, but pushing the limits too much can cause elimination issues that are much, much harder to fix than they are to prevent, so I stick with two even though odds are one box would be 'good enough' for my cats as long as I cleaned frequently enough.