Charles Ellicott Commentary Numbers 26:21

"And the sons of Perez were: of Hezron, the family of the Hezronites; of Hamul, the family of the Hamulites." — Numbers 26:21 (ASV)

Of Hezron ... — Judah had five sons, but since Er and Onan died childless, Hezron and Hamul were substituted in their place .