John Gill Commentary 1 Chronicles 2:4

"And Tamar his daughter-in-law bare him Perez and Zerah. All the sons of Judah were five." — 1 Chronicles 2:4 (ASV)

And Tamar his daughter in law bore him Pharez and Zerah
Through incest; they were twins, (Genesis 38:27–30) all the sons of Judah were five; who are before mentioned by name.