Judges 20:47

‘ But six hundred men turned and fled towards the wilderness, to the Rock of Rimmon, and they lived in the Rock of Rimmon for four months.'

Of the army that started out only six hundred identifiable men remained, although we can be sure that here and there stragglers escaped and found refuge somewhere. There are almost always some who escape even the worst massacres, to later describe what happened. The Rock of Rimmon was clearly inaccessible except individually and thus could easily be defended by a small force while they had supplies. The confederacy knew that they were there but could seemingly do little about it.