
(1) וידבר… במדבר סיני … באחד לחדש וגו׳ AND [THE LORD] SPOKE [UNTO MOSES] IN THE DESERT OF SINAI … ON THE FIRST DAY OF THE [SECOND] MONTH … [TAKE A SUM OF ALL THE CONGREGATION] etc. — Because they were dear to him, God counts them every now and then. When they went forth from Egypt He counted them (Exodus 12:37). When many of them fell as a consequence of their having worshipped the golden calf, God counted them to ascertain the number of those left (cf. Rashi Exodus 30:16). When he was about to make the Shechinah dwell amongst them (i. e. upon commanding them to make a Tabernacle), He again took their census. (Exodus 40:2) And shortly afterwards, on the first day of Iyar, He counted them.
Knowing the Rashi lets us compare and contrast commentary of the Ramban.
(ג) מִבֶּ֨ן עֶשְׂרִ֤ים שָׁנָה֙ וָמַ֔עְלָה כָּל־יֹצֵ֥א צָבָ֖א בְּיִשְׂרָאֵ֑ל תִּפְקְד֥וּ אֹתָ֛ם לְצִבְאֹתָ֖ם אַתָּ֥ה וְאַהֲרֹֽן׃
תפקדו אותם ענין פקידה זכרון והשגחה על דבר כלשון וה' פקד את שרה כאשר אמר (בראשית כא א) והוא פתרונו בכל מקום לא ימלט מהן איש על דעתי וגם פקדון מפני ששמירתו והשגחתו עליו וכאשר צוה למנות את ישראל יאמר תפקדו אותם לרמוז שלא יספרם רק שיתנו כופר נפשם מחצית השקל ובהם ישגיח וידע מספר העם
(1) ... Record (tifkedu) them: The matter of recording, remembering and guarding something - like the words (Genesis 21:1), "And the Lord remembered Sarah" - is the understanding of [pakad] in every place. Even [with a] pikadon (deposit) - it is [called that] because its guarding and supervision is upon [the holder]. And when [God] commanded to count the people of Israel, He said, "tifkod them," in order to hint not to [actually] count them, but rather that they should give a ransom for their lives, a half shekel - and through them, He would oversee and remember the multitude of the people.
(1) מבן עשרים שנה ומעלה, “from twenty years old and up;” from that age and up the males were considered as mature enough to serve in the army. Based on this verse, our Sages at the end of chapter 5 in Pirke Avot made the well known statement: בן עשרים לרדוף, “when having attained the age of twenty, one is fit to join the pursuit.” (2) כל יצא צבא בישראל, “all who are able to go to war on behalf of Israel;” the repetition of this word excluded the mixed multitude who had joined the people only at the Exodus.
The Mixed Multitude is a group of non-Jews who left the tyranny of Egypt with the Jewish People. In some commentaries they get a bad rap; they are known for having caused the sin of the Golden Calf for instance. What does Chizkuni's comment make evident about his attitude concerning the non-Jews who joined and lived with the Jewish People in the wilderness?

