Genesis 5: 1 – 5: This is the book of the generations of Adam. In the day that God created man, in the likeness of God made he him; Male and female created he them; and blessed them, and called their name Adam, in the day when they were created. And Adam lived an hundred and thirty years, and begat a son in his own likeness, after his image; and called his name Seth: And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters: And all the days that Adam lived were nine hundred and thirty years: and he died.
Genesis 6: 1: And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
The population of the pre-Flood world was of a far greater number than what we may at first imagine. The prophet Moses who wrote the Book of Genesis lists the generations of Adam all the way down to Noah. In this account we learn that Adam and his descendants were living for many hundreds of years, and in that time they had very large families made up of many sons and daughters. Even in examining the lineage of Cain after he was exiled from the presence of the LORD, we learn that he too had sons, and for that matter daughters, who further contributed to the rapid multiplying of human beings upon the face of the earth (Genesis 4: 16 – 18: And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. 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. And unto Enoch was born Irad: and Irad begat Mehujael: and Mehujael begat Methusael: and Methusael begat Lamech.).
Every son and daughter born of Adam and his named descendants in Genesis 5 would in the course of time have established families of their own, and because the longevity of man was very long in those days, these families would have been very large, for as can be expected over the course of hundreds of years, a married couple could easily have had many children. Based on this information and knowing that a space of 1656 years elapsed between the Fall in the Garden of Eden to the time of the Flood, we can calculate that the population of the pre-Flood world would have reached five billion people as a very conservative estimate. However, if we allow ourselves to be ever so slightly more generous in the number of offspring from a typical pre-Flood family (whilst still holding to a conservative, realistic simulation and allowing for the death rate), then we are looking at a possible world population of over seventeen billion people!