Noel B. Reynolds explains how the population sizes in the Book of Mormon are realistic:
Thoughtful students of the Book of Mormon have sometimes questioned the seemingly large number of Nephites who descended from Lehi’s original group. Critics have suggested on this basis that the Book of Mormon is demographically implausible.5 But it has now been shown that the size and fluctuations in Nephite numbers resemble the patterns of known historical populations. James E. Smith, one of the chief architects of the widely used Cambridge model for estimating historical populations, refutes the critics’ claim by comparing the Book of Mormon account with other ancient civilizations and by utilizing the Cambridge demographic model to demonstrate possible numbers of Nephites.6 He notes in passing that “if there is any hallmark of ancient historical records, it is their strong tendency to present [what might intuitively seem to be] puzzling, unrealistic, and inconsistent population figures.” Also, historical populations have generally experienced significant fluctuation and change similar to that depicted in the Book of Mormon.
Applying the Cambridge model with conservative assumptions about the growth of Nephite population, Smith calculated that the numbers in the text are on the high end of what would be predicted scientifically, but they remain plausible. For example, we know that “most of today’s six million French Canadians descend from about five thousand immigrant pioneers of the seventeenth century,” reflecting a much higher actual fertility rate than Smith assumes for his reconstruction of Nephite demographics. Relaxing any of Smith’s perhaps unduly conservative assumptions would move the numbers closer to the middle of the expected range. Additionally, if the Nephites or Lamanites absorbed any unmentioned populations, the numbers cease to be at all problematic. Because the demographic data in the Book of Mormon is incomplete, a precise picture of population sizes is impossible; however, as Smith concludes, “some plausible demographic inferences can be made, and the picture of Nephite population history that emerges is a realistic one.”
Joseph Smith went out on a limb when he included specific dates and population data in his translation of the Book of Mormon. Only in light of sophisticated analysis using tools far beyond the primitive Malthusian population projections of the early nineteenth century can modern readers appreciate how true to actual human experience such details in the Book of Mormon are– Noel B. Reynolds, “By Objective Measures: Old Wine in New Bottles,” in Echoes and Evidences of the Book of Mormon, edited by Donald W. Parry, Daniel C. Peterson, and John W. Welch (Provo, Utah: FARMS, 2002), Chapter 6
What are the odds that Joseph Smith would have created plausible population sizes like this?