2 Chronicles 29:21 Commentary - English Annotations on the Holy Bible by Matthew Poole

The number of seven is famous and customary in sacred matters, and is here used in regard of the vast numbers and various kinds of sins, the guilt whereof yet lay upon the kingdom, which was now to be expiated. Indeed, in case of one particular sin of ignorance done by the people, there was but one bullock to be offered, Lev 4:13,14, but here the sins were many and presumptuous, &c.

A sin-offering for the kingdom; to make atonement for the sins of the king, and the royal family, and the court.

For the sanctuary; for all the idolatry and uncleanness wherewith the temple had been polluted, which, as it had been cleansed naturally by the priests’ hands, so now was to be purged morally by sacrifices.

For Judah; for the sins of all the people of Judah.