פּירוש אויף במדבר 1:20

בכור ישראל. He had not lost his status in the books kept in heaven as he had repented his sin as testified in Genesis 35,22-23 commencing with the otherwise unnecessary words ויהיו בני יעקב שנים עשר, “that Yaakov’s sons remained twelve.” This was in contrast to Reuven’s birthright privileges here on earth having been revoked in favour of Joseph.

כל יוצא צבא, all who were able to go to war. The Torah repeats this comment every time the number of males in a tribe is given. This is because the Torah wants to tell us that amongst all the males over the age of twenty counted there was not even one who was physically unfit to go to war.

ויהיו בני ראובן בכור ישראל, “The members of the tribe of Reuven, Yaakov’s firstborn were, etc.” The Torah gave the number of members of the tribe of Reuven first, although in the list of encampments it listed Yehudah first, (2,3) seeing that Reuven was the biological firstborn. Shimon was listed next, in accordance with seniority of birth. Gad was listed next as he was the firstborn of Leah’s maidservant. Next the Torah lists the tribes which were grouped around the flag of Yehudah. Issachar and Zevulun who formed part of the army group are listed in that order as part of the camp of Yehudah. Subsequently the Torah lists Ephrayim followed by the tribes who were part of his camp, i.e. Menashe and Binyamin. Finally, the Torah lists Dan followed by Asher and Naftali who were part of his camp and flag. The tribes’ camps (Dan, Asher, Naftali last) are listed in the same order as here when the offerings on the occasion of the inauguration of the Altar in the Tabernacle are recorded in Numbers chapter 7.
The number of tribes listed and counted here are 12, seeing Levi is not included and two separate tribes are counted as Joseph. They were encamped facing the four directions East, South, West, North. Each army group comprised three tribes. Among the twelve tribes the tribe of Yehudah and Dan were the ones best known for their bravery; this is why they traveled at the head and at the rear of the Israelites respectively. Both of these tribes have been described at different times as גור אריה, “lion cub” (Genesis 49,9 and Deut. 33,22).
The word לגלגלותם, “according to their heads,” an expression used by the Torah only in connection with the tribes of Reuven and Shimon (verse 20 and 22), is one that applied to all the tribes. The reason that it was spelled out only with these two tribes may be that both of these tribes required atonement for sins committed by their respective founding fathers against Yaakov, and this is why the word פקודיו, “its numbered ones,” is also repeated in Shimon’s case, indicating that by now they were on a par with all the other tribes. Perhaps in the case of Shimon that word may not mean “numbered,” but may be a reference to the use of the word in the Ten Commandments, where G’d speaks about “remembering the sins of the fathers to the later generations” (Exodus 20,5). Verse 22 then ought to be translated as follows: “The matters remembered for the tribe of Shimon and his offspring according to their families and their fathers’ houses were, etc.” The words במספר שמות, “according to the number of names,” belong to what follows, to the word פקודיהם. The Torah wished to convey that actually the descendants of Shimon carried the burden of the sin of their founding-father and his part in the sale of Joseph (instigator) and by rights they should have paid for that unatoned for crime. Proof that Shimon had been the instigator of the violence committed against Joseph at the time is the fact that he was the only one detained by Joseph in Egypt after he permitted the other brothers to return and to bring Binyamin to Egypt on their next trip. (Compare Genesis 37,24, 42,24, Bereshit Rabbah 84,16).