Urinals should always come in odd numbers. Incidentally, since discovering this I have discovered that urinals generally come in odd numbers. The reason: The Code.
The Code dictates that unless there are none available, a man should always leave a space of one urinal between another urinating man. Sweet, sweet homophobia. So it follows thus that all places that have urinals should have them in odd numbers as to maximize efficiency. This way no two men need pee next to each other until The Code needs to be broken.
While one could create greater spacing between urinals, or put up walls to prevent wandering eyes, I venture that The Code would ultimately prevail and men would just space themselves farther while pissing. Wouldn't you?