(This is an excerpt from Mormon's Codex by John L. Sorenson.)
In the early days of their centuries-long conflict, when populations were relatively small, the Lamanite/Nephite wars must have been little more than raids (the first use of the term army or armies in the record was not until about 150 BC, Words of Mormon 1:13). The reported numbers of warriors involved and casualties suffered progressively increased over history. There was a respite from warfare for a couple of post-catastrophe, early AD centuries; then the numbers mounted again until the final slaughter of the Nephites late in the fourth century AD, at which time they comprised 230,000 men, likely plus women and children (Mormon 6:10–15). Are these credible numbers according to Mesoamerican history? Yes. The Quiché force opposing the Spaniards numbered 232,000 despite the fact that some groups abstained from the alliance. The Aztecs mustered a force of 400,000 in a fairly routine campaign against a nearby kingdom. More problematic is Alba Ixtlilxochitl’s account of central Mexican history, according to which a combined Aztec army at one point consisted of 700,000 men. Of the yet hazier past the historian said that in the last war of the “Tultecs,” which lasted three years and two months, a total (including women) of 5,600,000 persons were slain. Even if we skeptically and arbitrarily reduce that figure by 90 percent, the number would be of the same order of magnitude as the combined forces reported or implied in the Book of Mormon for the final battle at Cumorah.
It is unclear to what degree Book of Mormon characters engaged in military training beyond what they might have received in occasional militia service. Some Nephite leaders (e.g., Lehi and Teancum) appear in campaigns over a number of years, as though they might form a “permanent” military cadre. As noted above, Mormon was designated leader of the Nephite armies at age 15 (Mormon 2:1–2; earlier Moroni had become the Nephite commander at age 25), which suggests training at an even younger age. In Guatemalan history a Tzutuhil king prior to the Spanish conquest was succeeded on the throne by Rumal Ahaus, a youth only 19 years of age who became noted for prosecuting war “with all the eagerness peculiar to youth.” That makes the case of Mormon plausible (he himself observed, “Notwithstanding I being young, was large in stature,” Mormon 2:1). At least in general one could qualify for military leadership in Mesoamerica at a young age.
Mormon’s account gives details on force sizes and casualties, an indirect source of information on army size, at some points in such detail (e.g., Alma 2:19 reports 12,532 slain on one side and 6,562 on the other, and Mormon 2:9 reports an army of 44,000 against 42,000) as to show the reader that on-the-ground historical data had been utilized. That does not mean that all the size figures given were accurate, but the order of their magnitude doubtless is.
(John L. Sorenson, Mormon’s Codex [Salt Lake City: Deseret Book, 2013], 397–98).