Albert Barnes Commentary Numbers 23:1

"And Balaam said unto Balak, Build me here seven altars, and prepare me here seven bullocks and seven rams." — Numbers 23:1 (ASV)

Balaam, after the general custom of the pagan, prefaced his divinations by sacrifice. In the number of the altars, consideration was probably given to the number of the then-known planets. Yet Balaam evidently intended his sacrifice as an offering to the true God.