Rashi on TanakhNumbers 1

In the Sinai desert . . . on the first [day] of the [second] month, etc. Because they (the Israelites) are precious before Him (Hashem), He counts them all the time; when they went out of Egypt He counted them [Shemos, 12:37], and when they fell because of [the sin of] the golden calf He counted them to know the number of those who remained, when He came to cause His Presence to rest upon them He counted them. On the first day of Nisan the Mishkon was set up, and on the first day of Iyar He counted them.

According to their families. To know the number of each tribe

To the house of their fathers: Someone whose father is from one tribe and his mother from another tribe shall be maintained in his father’s tribe.

Individually Through the shekolim a half-shekel per head.

All those eligible for the army. This teaches us that no one is eligible for the army below the age of twenty.

Along with you, there shall be. When you count them, there shall be with you the leader of each tribe.

These are the ones called from the congregation. The ones who were invited to everything of importance in the congregation.

These men. Those twelve leaders [mentioned above, 5–15].

Who were designated: To him (Moshe) here by [their] names.

And they were registered by family ancestry. They brought their genealogical documents and witnesses who verified the status of their birth, this applied to each individual, to establish their kinship with the tribe.

However, the tribe of Levi you shall not count. The King’s legion is worthy of being counted separately. Another interpretation; the Holy One, blessed is He, foresaw that a decree would arise against all those who were counted from the age of twenty and above to die in the desert. He [therefore] said, “Let these [Levites] not be included, for they are mine, since they did not sin with the [golden] calf.”

You shall appoint the Levites. As Targum [Onkelos] translates, “appoint.” The word denotes appointment to authority over the subject of one’s appointment, as, “and let the king appoint officials.” (Esther 2)

[They] shall take it down. As Targum (Onkelos) translates, “they shall dismantle.” When they were going to depart in the desert from journey to journey they would dismantle it (the Mishkon) from its upright position and carry it to the place where the cloud came to rest. They camped there and set it up.

And the stranger who approaches: To [perform] this service of theirs.

Shall die: By the hand of heaven.

And each man at his own banner. According to the way that the banners are arranged in this book (i.e., Bamidbar, 2:3–31), three tribes to each banner.

So that there will not be anger. If you fulfill My commandments there will not be anger, but if not — if strangers will enter into this service of theirs — there will be anger, as we find in the episode of Korach, “. . . for the anger had emerged . . .” [17:11]