From Wiki:
Number of Coils:
Each additional coil adds friction to the knot, which makes the noose harder to pull closed or open. The number of coils should therefore be adjusted depending on the intended use, the type and thickness of rope, and environmental conditions such as wet or greasy rope. Six to eight loops are normal when using natural ropes. One coil makes it equivalent to the simple running knot.
The number thirteen was thought to be unlucky. Consequently, in myth, if not in actual practice, thirteen coils were found in a hangman’s noose, a foreboding sign for those convicted to be hanged.
Woody Guthrie sings of the hangman using thirteen coils:
Did you ever see a hangman tie a hangknot?
I’ve seen it many a time and he winds, he winds,
After thirteen times he’s got a hangknot.