Charles Ellicott Commentary 1 Samuel 27:2

"And David arose, and passed over, he and the six hundred men that were with him, unto Achish the son of Maoch, king of Gath." — 1 Samuel 27:2 (ASV)

The six hundred men. —This was the original number. They still formed the nucleus of the force, but the total number was now far larger. These “six hundred” each had their households; in addition to this, many a group of warriors, large and small, had already joined the now renowned standard of the future king.

Achish, the son of Maoch, king of Gath. —The same, we believe, as that Achish to whom David previously fled (see 1 Samuel 21:11), and identical with Achish, son of Maachah (1 Kings 2:39). This would involve the necessity of ascribing a fifty-year reign to this prince. (Such a lengthy reign is quite possible.) The whole of Philistia subsequently fell under King David’s rule. It seems, however, that he permitted, even after the conquest, Achish to remain in his old city of Gath, most likely as his tributary, thus, we may suppose, paying back the old debt of kindness to Achish.