They may be dominant but they're slightly less likely to piss on some odd corner or piece of furniture if they're annoyed about something. Neutered males can spray if provoked (the cat box needs to be cleaned daily or we sometimes encounter this vote of disapproval from our male cat).

The rule of thumb is one litter box per cat around the house, but in those multi-multi-cat households it would be a bit extreme. However, at least two, for various reasons. A door accidentally left shut is a prime example of why a backup somewhere else in the house is helpful.

SRS