After adding Joey to the household, I now have 5 indoor cats
but always had 5 litter boxes. They are all in the basement, 3
in the family room & 2 in the larger laundry/work shop area. I
too always thought the "one more than you have cats" idea
was probably a good idea. For my feral Momma & Baby kitty, I
have 3 litter boxes in the garage in different locations. Seems
to work pretty good that way.

p.s. I've never seen more than one cat use the box at a time.