According to the guide (0.9.2 version) it is 13, but I had 15 at the end of that, not sure how that can be remedied.
Mine was 14 at the end of 0.10. I just checked the code, and that appears to be the proper maximum for 0.10.
There is one that came from a 0.10 event, so makes sense that 0.9.2 max would be 13.
There is also some commented out code, that makes every single "apply sun cream" on the weekend patrol give a horniness point. Wouldn't be surprised if, in some version, that was the case, and maybe where you got your extra points? Maybe where I got an extra one as well, but I must have not saved afterwards. (Also, there is another repeatable horniness event with Amy I don't even remember, but maybe existed at one point, but is currently commented out.)
Anyway, to correct, activate console and:
$ horniness = 14
(or 13 if you haven't gotten the horny point from 0.10 yet)