A brief examination of the 14 rules used by Maimonides as criteria in compiling his list of the 613 mitzvos.
A mitzvah is a command to do (or not to do) something. There may be many details in how to accomplish a mitzvah, but they still all add up to a single mitzvah.
The Rambam uses the example of yibum and chalitzah, the obligation for a man to marry his deceased brother’s widow, or to release her. There are many factors determining who performs yibum, who performs chalitzah, who can perform either, and who may perform neither. Some of the circumstances apply to the brothers of the deceased and others apply to the widows of the deceased; anyone who has studied the tractate Yevamos knows how complex and intricate these circumstances can be. But each of these laws (e.g., such-and-such man may not perform yibum, only chalitzah, etc.) does not constitute a separate mitzvah. If they did, there would be more than 200 mitzvos pertaining to yibum! Clearly, this is not the case.
As with his other principles, the Rambam gives examples of predecessors who have erred in this matter when attempting to compile a list of the Taryag mitzvos. One example is found in the obligation to bring a sin offering for defiling sacred objects, which is a mitzvah. The details of this mitzvah include that one is to bring a female sheep or goat; if one cannot afford that, he brings two pigeons or doves; if one cannot afford THAT, then he brings a meal offering. But these are just details of the sin offering, not three separate mitzvos to bring animals, birds and flour.
Another example has to do with the case of a betrothed maiden who commits adultery. (Remember: “betrothal” is more than “engagement” – it’s marriage!) The girl receives the penalty of stoning unless she is a Kohein’s daughter, in which case the penalty is burning. These are not separate mitzvos; they are details of a single mitzvah – not to commit adultery.
The Rambam gives other examples. Tzaraas isn’t 11 mitzvos, it’s one mitzvah. The law of the person who kills accidentally isn’t 16 mitzvos, it’s one mitzvah. Not to use a blemished animal for a sacrifice is a single mitzvah; each kind of blemish – of which there are almost 70! – isn’t a separate mitzvah. The Rambam describes many such cases because this is apparently a source of confusion. He is most emphatic, however, that we not err in this matter.