Who knows fifty-one?
Please cite/link your sources, if possible. At some point in the next few days, I will:
Upvote all interesting answers.
Accept the best answer.
Go on to the next number.
51 are the mitzvos that the Rambam enumerates as related to avodah zarah (idolatry) - the most of any one section of halachos in his Mishneh Torah.