Genesis 5:3 - 5
ESV - 3 When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth. 4 The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
I believe it could've been hundreds. Earlier in Genesis, Cain fears anyone he sees. Who could he be afraid of? Likely, more children of Adam and Eve. God commanded Adam and Eve to be fruitful and multiply, and they did so.
Why were the other children left out?
According to scripture and my understanding.... Adam and Eve's family consisted of sons Cain, Abel, and Seth, plus a minimum of two other sons and two daughters, giving him a total of seven children. However Adam lived for 930 years so that is the minimum number of children, but does this seem reasonable?
Adam had 33 boys and 22 girls according to the Jewish historian Josephus.
I personally don't know.