By the 'golden sandbox rule', I'm a few sandboxes short.

Golden Sandbox Rule = One sandbox per cat + one extra, and one on every level of your home.

I have 8 cats and 6 sandboxes. The only 'accidents' I have is occasionally Jack will poop outside the box because he has constipation problems. We have been treating him, but 1-2 times a week he'll use the floor in front of the box. Bear & Cami will occasionally overspray the sides of the litterbox, but at least they are IN the box when they do it, never do that outside the box.

I have:

2 Extra large covered sandboxes in my rarely used foyer (downstairs).
1 Extra large covered sandbox in the spare room (upstairs).
1 Extra large covered and 2 Extra large uncovered sandboxes in our bedroom (upstairs).