Charles Ellicott Commentary 2 Chronicles 29:21

"And they brought seven bullocks, and seven rams, and seven lambs, and seven he-goats, for a sin-offering for the kingdom and for the sanctuary and for Judah. And he commanded the priests the sons of Aaron to offer them on the altar of Jehovah." — 2 Chronicles 29:21 (ASV)

Seven bullocks ... rams ... lambs. — For a burnt offering (‘ôlah). See the legal prescriptions respecting the sin offering (Leviticus 4:0). On the present extraordinary occasion, an extraordinary sacrifice was offered. Balak and Balaam offered seven bullocks and seven rams as a burnt offering (Numbers 23:1–2 and following).

And seven he goats, for a sin offering. — Compare Ezra 6:17; Ezra 8:35; Leviticus 4:23; Leviticus 4:28; and also 2 Chronicles 29:23. The reigning house and the sanctuary and the people had all contracted defilement during the late period of idolatry.

The priests the sons of Aaron to offer. — In careful accordance with the rule of the Torah.