Here's a point to ponder on this topic. There are several cities named in Genesis in the antediluvian world, so the population had to be at least several thousand if not more. We all remember that Noah built the Ark over the course of 120 years and he is described as a preacher of righteousness, and he was. He was spreading the word of a coming flood to people who had never seen rain. In the end, only eight people got on board the Ark and they were all from his family. Notice how the writer of Hebrews described what Noah did, "By faith Noah...prepared an ark for the salvation of his household...." (Hb. 11:7). The Ark wasn't big enough to accomodate the population of the entire world as it existed in those days. God knew that only eight would respond to the message and gave Noah the blueprints for the vessel that was designed for only the eight people who would climb onboard. Why is it so difficult to think that God knows the exact number of people who will respond to the gospel message and sent Christ to come and die for them?