As I've been told, five is the number of ribs, no more, no less. Five shall be the number of ribs you turn, the number of ribbing shall be five. Six shall not count, nor will four, excepting thou procedest to five. Seven is right out. Once the number five has been reached, being the fifth turn, then thou lobbest thy scud at your foe...
Seriously, though, I've been told that five turns is "traditional," but that it doesn't really matter how many you do as long as they're even and not crowded.