Albert Barnes Commentary 1 Samuel 31:6

"So Saul died, and his three sons, and his armorbearer, and all his men, that same day together." — 1 Samuel 31:6 (ASV)

All his men - This and similar expressions must not be taken too literally (Compare to 1 Chronicles 10:6). We know that Abner, Ish-bosheth, and many more survived the day of Gilboa.