Exode 30:12

When thou takest the sum of the sons of Israel according to their number, they shall give every man the ransom of their souls before the Lord when thou numberest them; that there may not be among them the calamity of death when thou dost number them.

The Holy One, blessed be He, commanded Moses that when he takes a census of the children of Israel, they should each give a soul’s ransom — half a shekel; and He further told him, And thou shalt take the atonement-money mentioned, and shalt give it for the service of the Tent of Meeting. From this Moses would understand that he was to count them now. And so indeed he did, as Scripture says, And the silver of them that were numbered of the congregation was a hundred talents etc. It was thus not necessary to state at length, “and now you should take their census and give the silver for the service of the Tent of Meeting,” for it is self-understood that he should count them now. The reason that He generalized the commandment — saying “when you will take their census you should do it in the prescribed way” — is in order to include in this general principle any time when a census was decided upon.
It appears to me that now [at this census, Moses] did not have to come to the people’s tents and count them, as he did in the census spoken of in the Book of Numbers,, 16). Ramban is here suggesting that the same was done with the other tribes as well. but he did it as our Rabbis have said through the [half] shekels they brought for the sacrifices; for he commanded them that all who know themselves to be from twenty years old and upward should give that sum, and they brought him the ransom as a voluntary offering together with all the other voluntary offerings every morning. That is why He said only, And thou shalt take the atonement-money, that is to say, “Behold, I have commanded you that when you count them they shall give their ransom, and now they will voluntarily give it and you should appoint it for the service of the Tent of Meeting.” This is the reason why it was not necessary now that Aaron and the princes [of the tribes] be present with Moses [at the taking of the half-shekels, as was required in the census spoken of in the Book of Numbers]. Do not object on account of the verse, This they shall give, every one that passeth among them that are numbered [from which you might think that here too, Moses actually counted the people], for the meaning thereof is “those that are eligible to pass.”
Now because it has not been explained here whether this is a commandment binding for all time or only for that particular period of when Moses was in the wilderness, David erred and counted the people without …

כי תשא, when Moses would assemble the Jewish people in order for them to set aside the donations for the building of the Tabernacle he was to use this opportunity to count them. The silver would be given for the work in connection with the Tabernacle that required use of this metal. In Exodus 38,25 we are told that a total of 100 talents of silver were used in the construction of the Tabernacle.

כי תשא..ונתנו איש כפר נפשו, the need to count human beings stems from the fact that human beings are not the same each time, i.e. ever since death was decreed upon mankind this reflects sin. Prior to sin man, who was meant to live indefinitely, would be the same at each count. The sages have said that no one dies unless he had committed a sin (Shabbat 55). It follows that mention of a head count of people is an oblique reminder of man’s sin, his guilt. This being so, it is most appropriate that at the time of such count he should pay some ransom on behalf of his soul, his life, so that he will qualify for atonement. This also explains why the poor must not give less nor the rich more, as a soul’s ransom is not calculated in terms of the amount of money offered to G’d.