Everyone is 5-15 syns according to everything I've seen in writing, but my SWC has told me I can have 20. Granted, that was when I started and was 24 stone, but I am 100% it was because I was a guy, not due to the amount I have to lose.
As I said though, I have only ever seen 15 in writing, and it is 15 I have as a maximum. I average 10-12 though.