Albert Barnes Commentary 2 Kings 18:23

"Now therefore, I pray thee, give pledges to my master the king of Assyria, and I will give thee two thousand horses, if thou be able on thy part to set riders upon them." — 2 Kings 18:23 (ASV)

The phrase translated “give pledges,” or “hostages” (from the margin), is perhaps best understood as meaning “make an agreement.” If you will “bind yourself to find the riders” (that is, trained horsemen), we will “bind ourselves to furnish the horses.” The suggestion implied that in all of Judea there were not 2,000 men accustomed to serving as cavalry.