How many stones did Elijah use to build the altar on Mount Carmel?

Answer: Twelve

Elijah took twelve stones, one for each tribe of the sons of Jacob, and built an altar in the name of the LORD.

“And Elijah took twelve stones, one for each tribe of the sons of Jacob, to whom the word of the LORD had come and said, “Israel shall be your name.” And with the stones, Elijah built an altar in the name of the LORD. Then he dug a trench around the altar large enough to hold two seahs of seed.”— 1 Kings 18:31-32 (BSB)