didn't cain and seth marry their sisters and have a gang of kids
serpentofeden .. dear serpentofeden ... You are a serpent.. so you never heard the story..
well The Bible does not give us a specific number.
Adam and Eve had Cain (Genesis 4:1), Abel (Genesis 4:2), Seth (Genesis 4:25), and many other sons and daughters (Genesis 5:4). With likely hundreds of years of child-bearing capability, Adam and Eve likely had 50+ children in their lifetime ., that is from OT but they lived for 900 or so years., if eve could give birth to triplets and quartets then in 900 years they can have 3600 kids.. and Imagine 3600 kid in another 10 years exponential birth of triplets.. it appears god of religions is a peeing peeping tom and made man and women to watch porno movie .

well let us read some genesis
1:26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
1:27 So God created man in his own image, in the image of God created he him; male and female created he them.
1:28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.
2:19 And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.
2:20 And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found an help meet for him.
2:21 And the LORD God caused a deep sleep to fall upon Adam, and he slept: and he took one of his ribs, and closed up the flesh instead thereof;
2:22 And the rib, which the LORD God had taken from man, made he a woman, and brought her unto the man.
2:23 And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.
2:24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
2:25 And they were both naked, the man and his wife, and were not ashamed.
Morale of the story that runs from these silly book sis INCEST..Incest and incest
After Adam and Eve had children, how did they proceed onto the third generation? The children must have slept with each other, or their own parents. How do you explain this to an inquisitive child without condoning incest? Incest was the order of the day, and would have been required for two or three generations. The same applies to the story of Noah's Ark. The children of Noah's family and his wife's family must have slept with members of their parent's family, or each other. Any further generations would again be faced with the same choice
well that is today's bible..