Verse Nu01-02 discussing the required census of the Jewish people states Take ye the sum of all the congregation of the children of Israel, by their families, by their fathers' houses, according to the number of names, every male, by their heads;

Rashi explains the two underlined words, family, heads by referencing other verses which illuminate these terms.

Rashi explains that By family means By tribe. Rashi derives this explanation by referencing the immediately following verses which describe census by tribe: Nu01-05:06 states And these are the names of the men that shall stand with you: of Reuben, Elizur the son of Shedeur. Of Simeon, Shelumiel the son of Zurishaddai. Rashi could also cite Nu26 which present a census by tribe and subfamily.

Rashi explains the requirement to count by heads by referencing Ex30-12:13 which describes the procedure of counting people by counting the half-dollars that they contribute. The verses explicitly state When thou takest the sum of the children of Israel, according to their number, then shall they give every man a ransom for his soul unto HaShem, when thou numberest them; that there be no plague among them, when thou numberest them. This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary--the shekel is twenty gerahs--half a shekel for an offering to HaShem. Verse Ex38-26, reviewing this procedure explicitly states a half dollar per head echoing the reference to counting by head in Nu01-02. The full text states a beka a head, that is, half a shekel, after the shekel of the sanctuary, for every one that passed over to them that are numbered, from twenty years old and upward, for six hundred thousand and three thousand and five hundred and