While Israel was living in that region, Reuben went in and slept with his father’s concubine Bilhah, and Israel heard about it. Jacob had twelve sons:
Jewish Publication Society of America Version (JPS1917) - Genesis 35:22 >And it came to pass, while Israel dwelt in that land, that Reuben went and lay with Bilhah his father's concubine; and Israel heard of it. Now the sons of Jacob were twelve: