The beginning of KI TISA is a portion of preliminaries. After the detailed instructions for the building of the Mishkan and the installment of the Kohanim (Shemot 25:1-30:10), we are now presented with a series of “appendices”:
• The giving of the half-shekalim.
• The Laver (kiyor).
• The oil of anointing and the incense.
• The appointment of the artisans Betzalel and Oholiav.
• The observance of Shabbat, even during the building of the Mishkan.
Delineated here are elements related to the building of the Mishkan that are not essential to its construction yet must be imparted before the actual building is undertaken. (This is even true of the Laver, where the main issue is the Kohen’s duty to wash his hands and feet before performing the Service.)

The first of these is the yearly commandment to give the half-shekalim:
And Hashem spoke to Moshe, saying: “When you take the count of the Children of Israel by their numberings, then each man shall give for Hashem a ransom for his soul when counting them; and there shall not be a plague amongst them when counting them. This shall they give, everyone who passes among those who are counted: half a shekel, by the holy shekel, a shekel is twenty gerahs; half a shekel is the offering to Hashem. Everyone, who passes through the census, from twenty years old and above, shall give the offering of Hashem. The rich shall not give more, and the poor shall not give less, than half a shekel, to give the offering of Hashem to atone for your souls. And you shall take the silver of the atonement from the Children of Israel and you shall give it for the work of the Tent of Meeting. And it shall be for the Children of Israel as a memorial before Hashem, to atone for your souls” (Shemot 30:11-16).

In the desert, the silver was used for the sockets and hooks (38:25-28), becoming a permanent “memorial before Hashem.” Henceforth, as long as the Sanctuary stood, the silver was spent for community offerings, as Sefer HaChinuch (ascribed to either R. Aharon HaLevi or R. Pinchas HaLevi of Barcelona, mid-13th Century) says (§ 105):
They would leave it all in one compartment in the Sanctuary, and from there they would take out funds to buy: the daily burnt-offerings, the additional offerings, and every sacrifice that was offered up on behalf of the public, and their libations; the salt with which the offerings were salted, the order of the piles of wood, the Showbread, and the wages for the one who made the Showbread; the Omer, the two loaves of bread, the red heifer, the goat dispatched, and the cloth strip of crimson.

At the same time, the half-shekels were used to take a census of the Children of Israel: by counting the coins, the size of the army is known. We thereby learn an ancillary law: never to count Jews directly, but only indirectly, as the Gra (R. Eliyahu of Vilna) says in Aderet Eliyahu:
“This is a commandment for all generations at all times whenever counting them.”

This is not a separate negative commandment, but a prohibition inferred from a positive commandment (lav haba michlal aseh). Rambam codifies this prohibition in Laws of Daily and Additional Sacrifices (4:4):
“It is forbidden to count Israel, except by means of another thing, as it says, and he counted them with sheep” (Shmuel I 15:4).
It applies even
• for the sake of a mitzvah, such as determining a minyan;
• when the count is not done by a king or leader; and
• for a part, rather than the whole, of Israel.
This law is also cited by Magen Avraham to Shulchan Aruch, Orach Chayyim 156:1. It is particularly relevant to the State of Israel, which has conducted censuses. (See R. Eliezer Waldenberg, Tzitz Eliezer 7:3; Techumin 4, p. 327 ff.; and R. J. David Bleich, Contemporary Halachic Problems, vol. III.)

Why is direct counting of Jews forbidden?

From Berachot 62b, it seems that the essential source of this law is our verse. However, as Yoma 22a demonstrates, the far-reaching nature of the prohibition is derived from such verses as the one cited by Rambam, as well as:
And the number of the Children of Israel shall be like the sand of the sea, which can not be measured or numbered (Hoshea 2:1).
This is an echo of the promises made to Avraham:
…that if a man would be able to count the dust of the earth, so would your seed be counted (Bereshit 13:16).
“Look, please, towards the heavens and count the stars … so will be your seed” (Ibid.,15:5).
Rashi to Divrei HaYamim I 27:24 suggests that these verses constitute the ultimate source of the prohibition.

Accordingly, counting Jews is defiance of Hashem’s promises. On the other hand, Rashi on our verse says that the “evil eye” (ayin hara) has influence over anything counted; this alludes to Taanit 8b, which teaches that blessing does not rest on anything measured or counted.

Sforno (R. Ovadia ben Yaakov Sforno, c. 1470-c.1550) explains that counting people highlights their transience, which is a direct result of their sins:
“It is therefore fitting that everyone give a ransom for his soul in honor of
G-d, may He be blessed, and He, being merciful, will atone for transgression (Tehillim 78:38), as it says, To atone for your souls (v. 15).”

R. Moshe Mat (c. 1551-c. 1606), in Hoil Moshe, Baer HaTorah, explains that a census undermines national cohesiveness:
“Since number indicates plurality, which is the opposite of unity, there will result from this division severe harm for the nation. …By means of the half-shekel it is shown that every one of them is greatly lacking until he unites and joins with the others.”

While learning about the building of the Mishkan, which unites Hashem with us, we are simultaneously taught the indispensable importance of unity within Hashem’s people.