Took in thirty daughters, i.e. took them home for wives to his sons. See Genesis 24:67, Genesis 31:50 Deuteronomy 21:12 2 Samuel 11:27.
Related Commentaries of Judges 12:9
Judges 12:9
9 And he had thirty sons, and thirty daughters, whom he sent abroad, and took in thirty daughters from abroad for his sons. And he judged Israel seven years.