Althought this block has several years, the question is still valid, as the commens mention, Adm and Eve had two sons, one killed the other and there was only one left. But the problem here is what Genesis 4:19 says:' 'And Lamech took unto him two wives: the name of the one was Adah, and the name of the other Zillah. And Adah bare Jabal: he was the father of such as dwell in tents, and of such as have cattle. And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. .. 23. And Lamech said unto his wives, Adah and Zillah, Hear my voice; ye wives of Lamech, ...... And 23 Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew. And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the Lord .'
https://my.bible.com/bible/1/GEN.4.17,19-20,23-2619
The only problem is Seth was the third son Adam and Eve, there was not a women with whom Cain could have children, so it seems that there were other sons or daugthers between Cain and Seth???