eighteen thousand. This was Abishai's command. In 2 Samuel 8:13 we have David's command; while in Psalms 60 (title, Psalms 60:1) we have Joab's share in the campaign (1 Kings 11:16). There is no contradiction, but independent and separate exploits. Abishai's number was 18,000. We do not know how long Abishai's took him. Joab's (which took him six months longer) was 10,000.
Related Commentaries of 1 Chronicles 18:12
1 Chronicles 18:12
12 Moreover Abishaig the son of Zeruiah slew of the Edomites in the valley of salt eighteen thousand.