Albert Barnes Commentary 2 Chronicles 13:9

"Have ye not driven out the priests of Jehovah, the sons of Aaron, and the Levites, and made you priests after the manner of the peoples of [other] lands? so that whosoever cometh to consecrate himself with a young bullock and seven rams, the same may be a priest of [them that are] no gods." — 2 Chronicles 13:9 (ASV)

Seven rams - A bullock and two rams was the offering which God had required at the original consecration of the sons of Aaron (Exodus 29:1; Leviticus 8:2). Jeroboam, for reasons of his own, enlarged the sacrifice and required it at the consecration of every priest.