I've had up to five cats using 2 litter boxes. We're currently at 4 cats to three boxes, but that may change back down to two once I have them all switched over to the CleverCat box (very closely resembles the cut-down storage container pictured above, but which has a sturdy platform that helps my older girl get in and out of it).
Largely, I think it depends on your cats' personalities. My ex had a flame-point siamese that would NOT, under any circumstances, share a litter box. If someone else dared soil his precious box and it not be scooped before he went in there next, he'd sniff it, turn and leave and go straight for our laundry hamper.
Our neighbors knew his name because they could hear me in the laundry room screaming, "OAKLEY!! BAAAAAD BOY!! NAUGHTY CAT!!" In the end, he went to a new home where he could be the only cat and we advised his new owners of his maintence requirements and that he needed to stay an only cat. The last I heard, he was doing great there with no problems what-so-ever.