John Calvin Commentary Numbers 26:19

"The sons of Judah: Er and Onan; and Er and Onan died in the land of Canaan." — Numbers 26:19 (ASV)

The sons of Judah were Er and Onan. Since the tribe of Judah was so numerous, Moses magnifies the greatness of God’s grace by this circumstance, that of the three sons whom he fathered, two perished in the land of Canaan, by whose loss he might have appeared to be condemned to perpetual sterility. But the climax of God’s unparalleled mercy was this, that although two of his children were born of an incestuous connection, they grew into so great a people.