TheSandTrap.com Top Picks
Actually, if you play 13 holes you must post an 18 hole round. If you play 7-12 holes post a 9 hole score.
USGA Handicap Manual 5-2b:
b. Posting a Score When a Complete Round is not Played
If 13 or more holes are played, the player must post an 18-hole score. If 7 to 12 holes are played, the player must post a nine-hole score. In either case, scores for unplayed holes must be recorded as par plus any handicap strokes that the player is entitled to receive on the unplayed holes. (See Section 4-2 and 5-1a .)
Example: A player with a Course Handicap of 30 stops playing after 16 holes because of darkness. Hole 17 is a par 3 and is the number 18 handicap-stroke hole. The player will record 3 (par) plus 1 handicap stroke for an X-4 on hole 17. Hole 18 is a par 4 and is the number 12 handicap-stroke hole. The player will record 4 (par) plus 2 handicap strokes for an X-6 on hole 18.