How many Amorite kings joined forces to attack Gibeon?

Answer: Five

The kings of Jerusalem, Hebron, Jarmuth, Lachish and Eglon, five Amorite kings, attacked Gibeon for making peace with Israel.

“So the five kings of the Amorites—the kings of Jerusalem, Hebron, Jarmuth, Lachish, and Eglon—joined forces and advanced with all their armies. They camped before Gibeon and made war against it.”— Joshua 10:5 (BSB)