Albert Barnes Commentary Exodus 24:4

"And Moses wrote all the words of Jehovah, and rose up early in the morning, and builded an altar under the mount, and twelve pillars, according to the twelve tribes of Israel." — Exodus 24:4 (ASV)

Twelve pillars - As the altar was a symbol of the presence of Yahweh, so these twelve pillars represented the presence of the Twelve tribes with whom He was making the covenant.