It should be an odd number on the neck with the forelock making it even...e.g 11 on neck plus the forelock!
Depends how thick her mane is though, I always go for 11 on mine, and use a little metal comb to measure them out. The overall picture looks better if each plait 'bunch' is the same size
As everyone has said 9 or 11 is the "correct" number according to showing. Thank goodness I don't show mine as I have to do approx 20 to keep them to a reasonable size!