HaShem said to Moshe: Gather 70 men from the elders of Israel... and take them to the Mishkan (Tabernacle), and have them stand there with you. I will come and speak to you there, and I will extend the spirit of prophecy that is upon you and place it upon them... (Bamidbar 11:16,17)
Shortly after a tragic fire killed "those at the edge of the Jewish encampment" (Bamidbar 11:1), Moshe was asked to appoint 70 elders to become temporary prophets and permanent leaders of the nation. Rashi (11:16) explains that they were to replace the original 70-man Sanhedrin, or Jewish supreme court, who expired in the above-mentioned fire. ("The edge" of the encampment that was burned, was the cutting edge.)
In truth, Rashi adds, the original Sanhedrin ought to have been punished earlier. The sin for which they paid with their lives occurred immediately before the Giving of the Ten Commandments, at which time the Sanhedrin was invited to climb Mt. Sinai along with Moshe, Aharon and Aharon's two sons, Nadav and Avihu, to experience a heightened measure of revelation of the Divine Presence (Shemos 24:11). Instead of approaching with the appropriate awe, however, they lightheartedly came to the appointment after a full meal, "like a slave that serves the king while biting on a piece of bread" (Rashi, Bamidbar ibid.) In order not to diffuse the Jews' joy of receiving the Torah, Hashem did not punish the elders of the Sanhedrin until this week's Parasha, after they had left Mt. Sinai on their trek towards the land of Israel.
Evidence that this was indeed the role of the 70 elders chosen in this Parasha can be gleaned from the Torah's description of how authority was bestowed upon them: "I will extend (Hebrew root: Atzal) the spirit of prophecy that is upon you...." When the Sanhedrin climbed Mt. Sinai to experience the Divine Presence, they were called "Atzilei" -- that is, the spirit of prophecy that was on Moshe *extended* upon them as well. Now that spirit would be extended to be placed upon others.
Although this explanation makes it obvious why 70 elders, and no more, were chosen for this experience, one early commentator (Moshav Zekeinim) points out that there was another factor that limited the number of "prophets" that Moshe appointed.
Moshe could only "extend" his source of prophecy to encompass the other elders while they stood with him "surrounding the Mishkan" (11:24). The dimensions of the Mishkan were 30 x 10 cubits (Shemos 26:16-23), and a person is generally one cubit wide (Gemara Sukah 7b). If so, exactly thirty elders could line up on either side of the Mishkan and another ten could stand at its western flank, making a total of 70 elders surrounding the Mishkan! Moshe stood by himself at the open eastern end of the Mishkan, so that the Divine spirit that emanated from the Mishkan would envelop him and extend to the elders.
Choosing 70 elders presented Moshe with a dilemma. How would he divide up the 70 fairly between the 12 tribes of Israel? As Rashi (11:26) explains, Moshe decided to choose six elders from each tribe (72 elders), and have them draw lots to see which two would not become leaders and share the prophetic experience.
The Torah goes on to describe how two of the chosen elders, Eldad and Meidad, decided in their exceptional humility that they were not fit to be prophets or leaders. They remained behind when the others went to join Moshe at the Mishkan. As reward for their humility, not only did Hashem grant them prophecy, He granted them an even greater level of prophecy than was experienced by the elders who stood with Moshe at the Mishkan! The other elders only prophesied for a short period time, while Eldad and Meidad did not cease prophecy.
The Gemara's interpretation would seem hard to reconcile with the verses. The Torah tells us clearly that Hashem extended the spirit of prophecy that was upon Moshe and placed it upon "the *70* elders... who became *temporary* prophets." Since two of the elders, i.e. Eldad and Meidad, became permanent prophets, shouldn't the Torah have said that only *68* became temporary prophets?
Upon further analysis, the answer to our question is obvious. Moshe was told to take 70 elders and place them around the Mishkan. When Eldad and Meidad did not come at the appointed time, Moshe took another two elders (from the 72 originals) instead of them. The spirit of prophecy rested on all 70 of those elders -- and also on Eldad and Meidad, who were back in the encampment, as well! All told, there were 72, and not 70, prophets besides Moshe on that day. (This appears to be the opinion of the Da'at Zekeinim 11:28,29 and Ohr ha'Chaim, 11:26)
It is appropriate indeed for Moshe to have appointed two "extra" prophets at this point -- two men who were even greater than the original 70, replacements for Nadav and Avihu, who overshadowed them.
