There are estimates that somewhere between 43-73 people traveled with Nephi to the New World. Of those, roughly a third split off to form the Lamanite group. So let's do some very basic math and see how many possible people we can come up with.
1) 30 to 50 Nephites
2) 5 to 7 children per generation
3) 3 generations each spanning 20 years roughly
(30 to 50) original Nephites
((30 to 50) / 2 * (5 to 7)) = 75 to 175 in generation 1
((75 to 175) /2 * (5 to 7)) = 187 to 612 in generation 2
((187 to 612) /2 * (5 to 7)) = 467 to 2142 in generation 3
This gives us a possible population when Jacob was talking of 759 to 2979. Now this does not take into account deaths or other factors, but Jacob could easily have been talking to hundreds if not thousands of Nephites by this point in time.
5 to 7 children that lived? That seems very high for that period of time. Here's some more info on the Book of Mormon population:http://www.mormonthink.com/book-of-morm ... population
This shows that the populations spoken of in the Book of Mormon could not have occurred unless the Book of Mormon people had a much, much, much higher rate of steady population growth than was ever achieved in the world until the industrialized era of the 20th century. Not to mention the fact that there were constant wars that were always killing off people.
It makes no sense unless the Book of Mormon peoples married Indians that already lived in the Americas but using that just brings up other problems as the numerous teachings of the prophets and the scriptures that show that the land was uninhabited until the Book of Mormon people came as it was saved for their inheritance. Then of course there's the flood that would have wiped out anyone that could have been here...