Quoting Commentary for I Chronicles 29:11

And in order to make the union between Israel and the Holy One, Blessed be He, resemble in every way an ordinary union, the Holy One, Blessed be He, also set a time limit for Israel of 10 months, from the first of Sivan, when they became betrothed through the Torah “which Moses commanded us,” until the first of Nisan, when the Tabernacle was erected and the union was completely fulfilled. During this time, Israel would adorn themselves with the 24 books [of the Hebrew Bible] and through this, they would be worthy to dwell in the Tent of Meeting, one dwelling for the two of them. And since the time of marriage is 30 days (see Ketubot 8), therefore the time extended until the first day of the second month in the second year. And if you wish to count from the verse and He gave to Moses when He finished [speaking with him] which was said in the month of Tammuz, after the 40 days when he brought down the tablets, then the count of 10 months would correspond exactly to the second month of the second year, for then the marriage was completed, and God wrote a ketubah [marriage contract] for her to know from what time it could be abrogated [by other] claimants. For the lands of the seven nations were pledged to them in the ketubah, so that if through [the Jews’] sins they would be exiled from the land, [the other claimants] could return and reclaim it from them, saying “our document’s date precedes [yours].” Therefore it says here in the Tent of Meeting for it was the time of the marriage, to dwell in a house. And so our Sages mentioned in Tractate Taanit (26b) these two times, and they expounded on the day of his wedding — this refers to the giving of the Torah, and on the day of the gladness of his heart — this refers to the building of the Temple. For in these two times the union was completed. And it mentions on the first day of the month for the reason that was explained. And in order that the union would be between similar partners, it says take a census [literally, lift the head] of the Israelites, in accordance with the Midrash (Bamidbar Rabbah 1:9) which says that just as it is said about the Holy One, Blessed be He, and exalts Himself as head above all (1 Chronicles 29:11), so “I have given you a lifting of the head,” etc. And what is the connection to here? It is only so that the union should be between similar partners, as it were.

שאו את ראש כל עדת בני ישראל, “take a census of the entire assembly of the Jewish people, etc.” The Jewish people do not experience an ascent of a spiritual kind unless they perform the laws of the Torah. When the counting of the Jewish people is described here by the word שאו instead of say, פקדו, this indicates that being counted at the command of G’d is a spiritual ascent. The word שאו combines within it two meanings, one the direct opposite of the other. It may mean that one’s head is being elevated in the sense of someone being promoted; on the other hand, the same word is also employed as depicting that someone is being beheaded, such as in the case of the chief baker of Pharaoh’s court whose execution Joseph foretold in Genesis 40,19.
We find a similar approach in Tanchuma Bamidbar 8 where the author paraphrases our verse as G’d saying: “I have not been as fond of any nation as I am of you, therefore I have elevated you to be the head just as I am the ‘head’ of all phenomena in this world.” We know the latter from Chronicles I 19,11: “Yours, Lord, are greatness, might, splendour, triumph, and majesty-yes all that is in heaven and earth; to You, Lord, belong kingship and preeminence over all.” G’d says that because of your relationship to Me, I have made you vis-a-vis the nations something similar to My position vis-a-vis all the creatures in the universe.” This is what the psalmist spoke about in Psalms 148,15: וירם קרן לעמו, “He has exalted the horn of His people.” When Moses speaks of G’d making the Jewish people supreme over all the nations of the earth, he has the same thought in mind.
When the meaning of the word שאו is “to decapitate,” this is what we read in Genesis 40,19. The Torah deliberately uses a word which may have either of these two meanings in order to warn the people that if they deserve it the census will be beneficial for them; if not, it may have fatal consequences.

FROM TWENTY YEARS OLD AND UPWARD, ‘KOL YOTZEI TZAVA’ (ALL THAT ARE ABLE TO GO FORTH TO THE HOST) IN ISRAEL. “This tells us that no one under the age of twenty goes forth to the host. ‘Se’u eth rosh’ [literally: ‘take the head’ and generally translated: take ye the sum] of all the congregation of Israel.¹⁶Verse 2. This is as one says to an executioner: ‘take that man’s head’.”¹⁷The meaning of this text which stems from Bamidbar Rabbah 1:9 will be explained further on by Ramban. The final quote is not found in our texts of Rashi, although Ramban quotes it from his commentary. The reason for its disappearance from all other texts of Rashi may well be its sharpness of expression which on first sight baffles the student. Ramban, however, recognizing that it is a genuine text which stems from Midrash Rabbah proceeds to explain it appropriately. This is Rashi’s language.
Perhaps the reason for this [law that a male under twenty years old was not liable to military service] is because he is not strong enough for war under the age of twenty, as the Rabbis have said:¹⁸Aboth 5:21. “Twenty is the age for pursuit.”¹⁹The Hebrew is lirdof, which is variously interpreted as: “for seeking a livelihood:” or “for seeking one’s life-pursuit.” Ramban here understands it in its literal sense — “to pursue [the enemy in battle].” But it may be that the meaning of the phrase kol yotzei tzava is “all who go forth to be assembled in the congregation,” because the young men do not take part in such an assembly of the people, and every gathering of the people is called tzava, as in ‘[litzvo] tzava’ (to be counted among the host) for the work of the Tent of Meeting;²⁰Further, 8:24. he shall return from the ‘tz’va’ of the work;²¹Ibid., Verse 25. with the mirrors of the ‘tzov’oth’ that ‘tzav’u’ (women that congregated together);²²Exodus 38:8. and similarly ‘tz’va’ (the host of) heaven;²³Deuteronomy 4:19. and all ‘tz’va’am’ (their host) I commanded.²⁴Isaiah 45:12. Therefore Scripture explains when speaking of men of war: ‘mi’tz’va’ (from the host of) the war;²⁵Further, 31:14. and the number of them reckoned ‘bi’tz’va’ (by the host) for the war.²⁶I Chronicles 7: 40. Scripture states here all that go forth to the host, similar to what it says, all that went out of the gate of the city.²⁷Genesis 34: 24. Ramban’s meaning is that the intention of the verses is: “all that are able to go,” even if they have not actually gone. It states, ye shall number them by their ‘hosts’ [in the plural], because they consisted of many hosts, since each and every tribe was a great host.
But as for Rashi’s expression when he wrote — “This is as one says to an executioner: ‘take that man’s head’” — it is not clear to me why the Sages should interpret the verse in this derogatory manner. If it is because [the people counted here] died in the desert [therefore He said, take the head …] whereas in the case of the tribe of Levi He said Number the children of Levi,²⁸Further, 3: 15. since they were not included in the decree [that they die in the desert] — [this cannot be so], for in the second census [taken] of those who were to come into the Land it also uses the same [expression], take the head of all the congregation of the children of Israel!²⁹Ibid., 26:2. But in the Agadah (homiletic exposition) of Vayikra Rabbah³⁰I did not find it there. But a similar exposition is found in Bamidbar Rabbah 1:7. See my Hebrew commentary p. 196, Note 26 for the full text. the Rabbis explain it as an expression of praise [for the people]: “The term se’u always means ‘greatness’, as it is written, Pharaoh ‘yisa’ (shall lift up) thy head, and restore thee unto thy office.³¹Genesis 40:13. Said the Holy One, blessed be He, to Israel: ‘I have given you an exaltation of the head, and I have likened you to Myself. Just as I am exalted above all inhabitants of the world, as it is said, Thine is the kingdom, O Eternal, and Thou art exalted as head above all,³²I Chronicles 29:11. so to you also I have given an elevation of the head, as it is said, Lift up the head of all the congregation of the children of Israel, thus fulfilling that which is said, And He hath lifted up a horn for His people,³³Psalms 148:14. and similarly it is said, and the Eternal thy G-d will set thee on high above all the nations of the earth. ' ”³⁴Deuteronomy 28:1.
I have found further in Bamidbar Sinai Rabbah that the Sages have stated:³⁵Bamidbar Rabbah 1:9. “Said Rabbi Pinchas in the name of Rabbi Idi: What is [the meaning of] that which is written at the beginning of this book, ‘se’u eth rosh’ (take the head)? It does not say: ‘lift up the head,’ ‘elevate the head,’ but it says se’u eth rosh, like a man who says to an executioner: ‘take this man’s head’. Here He gave a hint to Moses, se’u eth rosh, meaning that if the people are worthy they will become exalted, just as it is written, Pharaoh ‘yisa’ (shall lift up) thy head, and restore thee unto thy office;³¹Genesis 40:13. but if they are not worthy, they will all die, just as it is written, Pharaoh ‘yisa’ (shall lift up) thy head from off thee, and shall hang thee on a tree. ”³⁶Genesis 40:19. Thus the expression [se’u eth rosh which, as Rashi explained, is used in connection with execution] is to be interpreted according to the intention [of the speaker] — in a good way for those who are good.³⁷See Psalms 125:4: Do good, O Eternal, unto the good. Thus Pharaoh said unto the chief executioner: “Take the chief butler and lift up his head and restore him to his office.” But as for such as turn aside unto their crooked ways, the Eternal will lead them away with the workers of iniquity (ibid., Verse 5). Thus in the case of the chief baker, the same expression signified his downfall. In short, the expression of the Midrash which Rashi quoted can be interpreted either for bad or for good. And since it is an expression of elevation etc. And since it is [also] an expression of elevation, and it is used [here] in the first census, He used the same expression in the second census.²⁹Ibid., 26:2.