Could it possibly be a way for the manufacturers to sell more litter? Naaaaahhh…. 🤨When I had all four girls, I had two litter boxes. When I had three girls, I had two, but they mostly used one of them, not the other. So when I moved, I only put one out. I scoop 2-3 times a day, and it works for them. This "rule" that you have to have one for each cat and a spare is really ridiculous. That means when I had four girls I should have had 5 litter boxes. I can see putting extra boxes out if the cats are new or there's a problem, but besides that, it's a judgment call.
The "experts" say it too. 🙀I'm not sure how it started, but in my experience, the "rule" comes from fellow cat owners.