The largest armies fielded at the time by the biggest empires (for example, Egypt or Assyria) had between 20,000 and 50,000 troops. It is very unlikely that Hadadezer’s army was this size. Most likely the large number of captive troops reflect an ancient Near Eastern literary device in which numbers are exaggerated in order to glorify a particular deity. The large numbers are therefore meant to draw attention to the might of David (as he was helped by the Lord). 1 By capturing his troops and military equipment David drastically reduced the military threat to Israel from the north.2
4 And David took from him 1,700 horsemen, and 20,000 foot soldiers. And David hamstrung all the chariot horses but left enough for 100 chariots.