Charles Ellicott 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)

Build me here seven altars. —The patriarchs in ancient times, just as their pious descendants did after the Law was given, never erected more than one altar in one place. A plurality of altars was the badge of idolatry. Hengsten-berg cites several instances as proof that the ancients customarily resorted to sacrifice and conjuration to avert calamity and produce prosperity (History of Balaam and his Prophecies, p. 392). The number seven was regarded as significant among the Greeks and Romans, as well as among the Israelites.