וַיִּֽהְיוּ֙ כָּל־הַפְּקֻדִ֔ים שֵׁשׁ־מֵא֥וֹת אֶ֖לֶף וּשְׁלֹ֣שֶׁת אֲלָפִ֑ים וַחֲמֵ֥שׁ מֵא֖וֹת וַחֲמִשִּֽׁים׃
всего пронумерованных было шестьсот тысяч три тысячи пятьсот пятьдесят.
Or HaChaim
Our question is reinforced when we consider a comment in Pessikta Zutra according to which the words ויהיו כל הפקודים in Numbers 1,46 are proof that as long as the Israelites were occupied with the work pertaining to the Tabernacle not a single one of them died. This comment seems very far fetched if it is based only on the expression ויהיו, "they remained (alive)." When we look at the numbers presented here we are bound to conclude that more of the people died after the people commenced with the work of the Tabernacle than during the episode of the golden calf!
Or HaChaim
ובני ישראל היוצאים מארץ מצרים, and the children of Israel who took part in the Exodus from Egypt. It is quite unclear what the Torah intended with these words. Rabbi Avraham ibn Ezra suggests that the Torah meant that many of the people who were being counted now had actually taken part in the Exodus. Even if he is correct, why would the Torah consider it necessary to tell us about this at this point? Moreover, who needed Rabbi Ibn Ezra to tell us this? It is quite clear that many of the people who were below the age of twenty at the Exodus must have been included in this census! After all, the decree that the generation of the Exodus were to die in the desert after the debacle with the spies had applied only to people who had already reached the age of twenty either at the time of the Exodus or at the time the spies came back with their report! Perhaps it was the intention of the Torah to inform us that the result of this census produced a number equal to that of the census at the time the Israelites left Egypt as opposed to the number which resulted at the time the Israelites were counted at the beginning of the Book of Numbers. At that time the total amounted to 603,550 whereas this time the total was 601,730. In other words, the number was slightly lower than it had been some 38 years ago. The message then would be that Moses was not obligated to hand over to Joshua the same number of fighting men he had been able to muster at the time when he counted them earlier. The important thing was that he could show that he did not hand over fewer men than at the time of the Exodus. The census did prove this. [You will recall that I pointed out that the number at the Exodus included at least approx. 20.000 Levites so that Moses handed over far more men than he had at the time. Ed.] Another meaning of the words in question may be this: The number i.e. the census of Israelites on this occasion followed the same pattern as it had when they left Egypt, i.e. according to the houses of the respective tribal heads; Reuben according to the families of Chanoch, Phalu, Chetzron, and Carmi. Similarly, Shimon according to its tribal heads, i.e. Nemuel, Yamin, Yachin, Zerach and Shaul. This was in contrast to the count at the beginning of the Book of Numbers where the families of the tribes are not featured separately at all.
Radak
ויהיו כל ימי אדם, the reason why the Torah summed up once more the total of the number of years Adam lived since his creation, although anyone who is not a simpleton could have figured this out from the data already provided by the Torah, is in order that the scribe should not make a mistake. Once both the individual numbers and the sum total is provided one can double check the total and no mistake will be made. When the number of the soldiers in the army of Israel is recorded, an individual number being supplied for each tribe, the Torah nonetheless troubles itself to spell out the grand total of all these numbers combined. Compare (Numbers 1,46 as well as 26,51).
Rashi
ונתת אתו על עבדת אהל מועד AND THOU SHALT GIVE IT FOR THE SERVICE OF THE APPOINTED TENT — From this statement you may learn that he (Moses) was commanded to take their census when they began to contribute towards the building of the Tabernacle, after the incident of the golden calf, because the pestilence had befallen them, as it is said, (Exodus 32:35) “and the Lord plagued the people”. A parable: To what may this be compared? To a flock that is dear to its owner upon which there fell pestilence. As soon as it ceased he said to the shepherd, “I beg of you, count my sheep and ascertain how many of them are left”. He did this to show that it (the flock) was dear to him (Midrash Tanchuma, Ki Tisa 9). — It is, however, impossible to say that the census mentioned here is identical with that spoken of in the Book of Numbers (1:1), for there (in Numbers 1:1) Scripture states, “[And God spake to Moses …] on the first day of the second month … [Take ye the sum of all the congregation of the children of Israel etc.]”, whilst the Tabernacle was set up on the first day of the first month as it is said, (Exodus 40:2) “On the first day of the first month shalt thou set up [the Tabernacle etc.]”, and from this census (that mentioned in this paragraph) — from the shekels obtained by it — the sockets used for its boards were made, for it is said, (Exodus 38:27) and of the hundred talents of silver were cast [the sockets of the sanctuary]”. Consequently you learn from this that two censuses were taken, viz., one at the beginning of their contributing towards the Tabernacle, after the Day of Atonement (when Moses first commanded the people to engage in the construction of the Tabernacle; see Rashi on Exodus 33:11, last sentence), in the first year, and the other in the second year, in Iyar, after the Tabernacle had already been set up. If you ask, however: is it at all possible that on both of these occasions the number of the Israelites was exactly the same, viz., 603,550, for in the account of how the silver of those that were numbered of the congregation was used (Exodus 38:27) it is so stated, and in the Book of Numbers (1:46) exactly the same is stated, “Even all they that were numbered were six hundred thousand and three thousand and five hundred and fifty”; were not these censuses taken in two different years, and surely it is impossible that there were not at the time of the first census people nineteen years old who accordingly were not counted and who became twenty years old in the second year when the second census was taken and this must have added to the total?! The reply to this question is: As far as the years of men’s ages are concerned they were counted in the same year, but reckoning from the time of the exodus from Egypt they were two different years in which the censuses were held. For when we speak of a period beginning with the exodus from Egypt, which took place in Nisan, we calculate from Nisan, as we have learned in Treatise Rosh Hashanah 2b. Consequently the Tabernacle was being built in the first year and was erected in the second year, because a new year began on the first of Nisan following the exodus. The years of people’s ages, however, were counted according to the era of the creation of the world which begins with Tishri, consequently from this point of view both censuses took place in the same year: the first in Tishri after the Day of Atonement when the Omnipresent became reconciled with Israel to grant them His pardon and when they received the command regarding the construction of the Tabernacle, and the second on the first of Iyar, so that those who were only 19 years old after the Day of Atonement when the first census was taken, were not yet regarded as twenty in Iyar when the second census was held even though they were born between Tishri and Iyar.