Genesis 35:22

While Israel lived in that land , Reuben went and lay with Bilhah his father’s concubine . And Israel heard of it. Now the sons of Jacob were twelve .

While Israel was living in that region, Reuben went in and slept with his father`s concubine Bilhah, and Israel heard of it. Jacob had twelve sons:

And it came to pass, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father's concubine : and Israel heard it . Now the sons of Jacob were twelve :