John Gill Commentary 2 Samuel 2:15

"Then they arose and went over by number: twelve for Benjamin, and for Ish-bosheth the son of Saul, and twelve of the servants of David." — 2 Samuel 2:15 (ASV)

Then there arose and went over by number twelve of Benjamin , &c.] Whom Abner had picked out of that tribe, being his own, and whom he knew to be stout and courageous men, and closely attached to him. It seems by this as if Abner's men,

[which pertained to] Ishbosheth, the son of Saul , passed over the pool of Gibeon unto Joab's men; so forward were they to engage in this duel, and it makes it still more appear that they were the aggressors:

and twelve of the servants of David ; of his army under Joab, whom Joab either selected, or they, offered themselves as willing to engage with the twelve that were come over.