Can someone help me with a math issue involving the Bilble?

The story goes that God created Adam and Eve which would put (2) two people on the planet.

Adam and Eve had Cain and Abel, Which brings the total to (4) four people on the planet.

Cain became jealous and killed Abel, Dropping the total to (3) three people on the planet.

But then the story turns and talks about a mark being put upon Cain that no harm will come to him, he then runs off and marries some chick in the land of NOD.

Ok, so who are these people that are gonna hurt Cain, where did they come from, who is the chick he married, where did she come from,and why is there no information about another tribe city or colony? A preacher once tried to tell me that this was many many years later, and these people were the offsprings of Adam and Eve. NOT POSSIBLE. for they would have been Cains Brothers and Sister yet there is no information of their births and tribulations, like Cain and Abels, and if it is then why tell about the two brothers but leave out all the other Important information, like the colonization of NOD?