The standard advice is one box per cat plus one, so two cats get three, placed in different areas rather than lined up as a single station.
Cats treat side-by-side boxes as one resource, and a dominant cat can guard the approach, forcing others to hold urine or find alternative spots.
Multi-floor homes need at least one box per floor so cats avoid long trips at night or when unwell; seniors and kittens are especially sensitive to distance.
Too few dirty boxes is a common cause of out-of-box elimination, and adding boxes and scooping fixes most environmental cases, though persistent issues need a vet.
