Did Gideon have seventy sons or seventy-two?

The figures fit once we see how ‘the seventy’ is used. Judges 8:30 gives Gideon seventy sons by his wives, and 8:31 adds one more, Abimelech, by a concubine in Shechem: seventy-one in all. From then on ‘the seventy sons of Jerubbaal’ is the name for the group of Gideon’s sons by his wives. Abimelech uses it (9:2), so do Jotham (9:18) and the narrator (9:24, 56). Abimelech killed the group, all but one. Verse 5 says he slew them, “threescore and ten persons, upon one stone”, and in the same breath adds “notwithstanding yet Jotham the youngest son of Jerubbaal was left; for he hid himself.” Jotham, the one who escaped, still speaks of seventy slain (9:18). Plainly ‘seventy’ here is the round figure for the brothers as a body. Sixty-nine were killed, Jotham escaped, and Abimelech was the seventy-first son.

Keil and Delitzsch and Gill read the passage this way. The New Testament uses a group name in the same manner. Paul says that the risen Jesus was seen “of the twelve” (1 Corinthians 15:5), though Judas was dead and only eleven remained. The count of seventy-two comes only from treating ‘seventy killed’ as an exact figure and then adding back the one son whom the same verse says was not killed.