Albert Barnes 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)

Hezekiah began his restoration of the worship of the LORD with an unusually comprehensive sin-offering. This offering included the four main kinds of sacrificial animals, with seven of each kind, and was intended to atone for the sins—both conscious and unconscious—of the king, the priests, the people of Judah, and the people of Israel.

After these rites of atonement were completed, he proceeded with the burnt offering (2 Chronicles 29:27).