The standard guidance is one litter box per cat, plus one extra, placed in different locations rather than lined up in a row. Most litter box problems in multi-cat homes trace back to ignoring this simple rule.
Location matters as much as quantity. Cats generally avoid boxes near loud appliances, in high-traffic hallways, or too close to their food and water. If one cat seems to be guarding access to a box, that’s often the real cause of accidents elsewhere in the house, not a training issue.
Scooping daily and a full clean on a regular schedule solves more litter box aversion than any special litter or box design.