smiting = his smiting. This is David's exploit. In 1Ch_18:3, 1Ch_18:12 it is Abishai's command, while in title of Psa 60 (Psa_60:1) it is Joab's share in the campaign (1Ki_11:16).
eighteen thousand. This is the total. Joab's share was 12,000, and took six months longer. Compare 1Ki_11:13, 1Ki_11:16.