We have no way of knowing the amount of children Adam and Eve have in the Bible.
The Bible only records those of the family line of Seth, of which Noah and his family were from.
The Bible just says the following about Adam’s other children.
- (Genesis 5:3-5) 3 Adam lived for 130 years and then became father to a son in his likeness, in his image, and he named him Seth. 4 After becoming father to Seth, Adam lived for 800 years. And he became father to sons and daughters. 5 So all the days of Adam’s life amounted to 930 years, and then he died.
Considering that there were no means of birth control, Adam and Eve most likely had “stair step” children, maybe yearly. If so, by the time Seth was born, they could have easily had over 100 children.
Since only the first born, Cain, is mentioned, and Abel, the one that was murdered by him, it doesn’t mean that they didn’t have children in between them. Cain was concerned that the others would seek revenge on him for killing Abel. It is possible that dozens of Abel’s brothers would seek this revenge.
Seth was most likely the first male child born after Abel’s murder.