13
Kenan lived 840 years after Mahalalel was born. He also had other sons and daughters.
Genesis 5:13 Meaning and Commentary
Genesis 5:13
And Cainan lived, after he begat Mahalaleel, eight hundred and forty years The Septuagint has seven hundred and forty, which, added to the years given him before, makes the same sum:
and begat sons and daughters; as his progenitors did.