Here in Alma 2:19 we have record of 19,094 men killed in one day of civil war. This might suggest that the land of Zarahemla was somewhat populated at this time. At least it was apparently much more populated than the land of Lehi-Nephi at the time of Zeniff in which 3043 + 279 men were killed in one day of battle (Mosiah 9:18). For those who assume that the land of Zarahemla was located in the Chiapas valley of southern Mexico, there are over 400 archaeological sites in the region that date to this time period.
According to John Sorenson, all these people were "Nephites" politically speaking; the account does not talk about Lamanites at all. . . . It is reasonable that not over half the combatants were slain, which means that at least 40,000 warriors were involved, and perhaps somewhat more. Various studies of ancient warfare suggest how to translate that figure to total population. The ratio usually believed to apply is one soldier to about five total inhabitants. Using that figure, we may conclude that the total population of those "who were called Nephites" was 200,000 or more. Certain peoples in highland Guatemala shortly before the time of the Spanish conquest are reported in traditions to have fought with armies of 60,000, 80,000, and even up to 200,000 on one side, for decade after decade. [John L. Sorenson, An Ancient American Setting for the Book of Mormon, F.A.R.M.S., pp. 193, 195]
Note* To those who propose a geographical setting anywhere else in the Americas, providing evidence for this kind of population density at this period in time can be a problem. [Alan C. Miner, Personal Notes]