A brief examination of the 14 rules used by Maimonides as criteria in compiling his list of the 613 mitzvos
The statement of the Talmud (Makkos 23b) that God gave Moshe 613 mitzvos at Sinai expounds upon Deut. 33:4, “Moshe commanded us the Torah…” According to the Rambam, the “Torah” referred to in that verse only includes those commandments intended for posterity. The reason is because the verse continues, “an inheritance for the congregation of Yaakov,” and something can only properly be called an inheritance if it is transmitted from one generation to the next.
Omitting “one-shot” commandments from consideration would eliminate everything required only of the generation of the Exodus, such as not leaving over manna (1) or placing a molten snake on a pole (2). If one were to include commandments to the generation of the Exodus, where would the line be drawn? Would the obligation not to let one’s flocks graze too close to Mount Sinai be included (3)? This could be taken to ridiculous extremes, as the Rambam tells us that instructions given to this generation alone exceed 300!
Obviously, all such “one-shot” mitzvos cannot be included, therefore, the Rambam says, none are properly included. So commandments to be performed when the nation first entered Israel, such as pronouncing the blessings and curses on Mt. Gerizim and Mt. Eival (4), are not part of the 613 mitzvos.
Sources: (1) Exodus 16:19; (2) Numbers 21:8; (3) Exodus 19:24; (4) Deut. 21:11.