This is quite tough question as there aren’t any Biblical comments on this, yet we can reach to 2 conclusions:
(i) God could have created more humans beings just like HE made Adam & Eve.
- When God coursed Cain for killing Abel, he asks God that he can’t bear the course and anyone finding him wandering alone can kill him. This gives the clue that there were many other human beings too.
You are driving me off the land and away from your presence. I will be a homeless wanderer on the earth, and anyone who finds me will kill me.
“No, If anyone kills you, seven lives will be taken in revenge.” So the Lord put a mark on Cain to warn anyone who met him not to kill him. Genesis:4:14-15
(ii) Adam and Eve’s grandchildren came as a result of their children marrying among themselves.
- Bible does tell that, Adam lived up to 930 years and had other children.
Scientifically (ii) could arise serious genetic diseases due to “inbreeding” and fall of that generation. I would stick to (i) but its my personal view.