Adam Clarke on Numbers 23:1

Build me here seven altars, etc. - The oxen and the rams were such as the Mosaic law had ordered to be offered to God in sacrifice; the building of seven altars was not commanded. Some think that these seven altars were built to the seven planets: this is most gratuitously said; of it there is no proof whatever; it is mere trifling, even with conjecture. As seven was a number of perfection, Balaam chose it on this occasion, because he intended to offer a grand sacrifice, and to offer a bullock and a ram upon each of the altars; the whole to be made a burnt-offering at the same time. And as he intended to offer seven bullocks and seven rams at the same time, it could not be conveniently done on one altar, therefore he ordered seven to be built. We need go no farther to find out his reasons.