John Gill Commentary 2 Samuel 18:7

"And the people of Israel were smitten there before the servants of David, and there was a great slaughter there that day of twenty thousand men." — 2 Samuel 18:7 (ASV)

Where the people of Israel were slain before the servants of
David
That is, the people of Israel that were under Absalom, these were beaten by David's army:

and there was a great slaughter that day of twenty thousand [men] ;
including both those that fell in the field of battle, and that were slain in the pursuit; and this is to be understood only of Absalom's party.