According to the Book of Mormon, when Lehi left the old world (1 Nephi 18:8) his group consisted of less than twenty people. Yet, nineteen years later this small band had so prospered in the land that they built a temple which “manner of the construction was like unto the temple of Solomon; and the workmanship thereof was exceedingly fine” (2 Nephi 5:16).
Solomon’s Temple required 30,000 Israelites, 150,000 hewers of stone and carriers, 3,300 supervisors (1 Kings 5:13-16), and about 7 years of construction (1 Kings 6).
The Book of Mormon doesn’t indicate there were other people inhabiting the Americas in 600 BCE and tends toward the complete opposite. It further tells us that in less than 30 years from arriving on the continent, the population had multiplied so rapidly that they divided into two great nations.
Even the most rapid human reproduction could only have a few dozen people in this amount of time. Most of them would still be infants or children, and about one-third of them would be elderly.