The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters.
4. And the days of Adam after he had begotten Seth . In the number of years here recorded we must especially consider the long period which the patriarchs lived together. For through six successive ages, when the family of Seth had grown into a great people, the voice of Adam might daily resound, in order to renew the memory of the creation, the fall...
The first words of the chapter are the title or argument of the whole chapter: it is the book of the generations of Adam; it is the list or catalogue of the posterity of Adam, not of all, but only of the holy seed who were the substance thereof (Isa 6:13), and of whom, as concerning the flesh, Christ came (Rom 9:5), the names, ages...
Commenting on Genesis 5:1-5
And the days of Adam, after he had begotten Seth, were eight hundred years,.... The Septuagint version is seven hundred; for having added one hundred years more the should be, to the years of his life before the birth of Seth, here they are taken away to make the number of his years complete: and he begat sons and daughters; not only after the birth...