The phrase "from Dan to Beersheba" ( Judges 20:1) became a common expression during Israel"s united monarchy and indicated the whole of Israel. Gilead refers to the part of Israel east of the Jordan River. The Mizpah referred to here ( Judges 20:1) was the one in Benjamin just five miles north of Gibeah, not Mizpah of Gilead. Three times in this pericope the writer used the phrase "as one man" ( Judges 20:1; Judges 20:8; Judges 20:11). This is one of the rare instances of Israelite solidarity during the Judges Period. Here they unanimously chose a plan that lacked divine initiative. At other times they did not cooperate to fulfill the revealed will of God (cf. Judges 5:15-17; Judges 8:1-3; Judges 12:1-6; Judges 15:11). [source][source][source]
By casting lots to see how they should proceed against Benjamin ( Judges 20:9), the tribes were dealing with Benjamin as they had dealt with the Canaanite towns they had attacked. God did not tell them to deal with their fellow Israelites this way (cf. Deuteronomy 13:12-18). They were now battling their brethren as they had engaged their enemies ( Judges 20:18; cf. Leviticus 19:18). [source][source][source]
"Some comment must be made regarding the large numbers in this chapter. The discussion centers around the translation of the Hebrew word eleph. This word often is translated thousand but can also mean a family, clan, or military unit of fighting men (such as a squad of ten to twenty soldiers). The twenty-six, twenty-two, eighteen, ten, should not be thought of as so many thousand men but as so many units of men, each unit consisting of somewhere in the neighborhood of ten to twenty fighting men each. (A unit of ten is mentioned specifically in Judges 20:10.) This interpretation does not detract from the authority of the Scriptures in any way. It simply attempts to understand what the Bible actually says. Certainly it places the other numbers in the chapter in a reasonable context." [1][source]
I see no reason to reject the traditional translation of eleph as "thousand" in this context (cf. Numbers 26:41). [1]9 [source]