According to the theory of Jerry Ainsworth, the number 24 was significant to Mormon. It was a reoccurring and symbolic number in Nephite and Jaredite history. (See the commentary on Mormon 6:15.) If one keeps to that concept, one must contemplate why there were only 23 divisions of 10,000 soldiers who fought at the final battle at Cumorah. Where was the twenty-fourth division?
According to Ainsworth, a possible answer is that once all the Nephites had gathered to Cumorah, Mormon sent a regiment of soldiers northward with the unfit for battle. The lands northward had been colonized by the Nephites and people of Ammon for over four hundred years. These were kindred peoples to the Nephites, people who understood conditions in the land southward--conditions from which they themselves had fled. Surely, these migrants would provide refuge to any retreating and surviving Nephites. Perhaps Mormon also felt responsible for defending such migrant communities in the north countries from the Lamanites, especially those who were "peaceable followers of Christ" (Moroni 7:3).
The army Mormon may have sent to protect these people most likely was comprised of a commander, with "ten thousand" soldiers. They would have made up the twenty-fourth regiment of Nephites that had gathered at Cumorah. It seems unlikely that those soldiers would have been Mormon's elite troops. In all probability, they would have been inferior, less trained, and possibly aged and less capable. It was a practice in ancient Israel to send away from the battle the faint hearted and any who would hinder the fight (see Deuteronomy 20:8).
Mormon refers to such inferior troops when he writes to his son Moroni after the battle at Cumorah. He says, "The army which is with me is weak. . . . They are without order and without mercy. Behold, I am but a man, and I have but the strength of a man, and I cannot any longer enforce my commands" (Moroni 9:17-18)
Another possible example involves one of Mormon's commanders (possibly the commander of the twenty-fourth division) who had taken provisions intended for those unfit for battle. His army had begun to run from the Lamanites without concern for those whom they were charged to protect:
There are many widows and their daughters who remain in Sherrizah; and that part of the provisions which the Lamanites did not carry away, behold, the army of Zenephi has carried away, and left them to wander withersoever they can for food; and many old women do faint by the way and die. (Moroni 9:16).
[Jerry L. Ainsworth, The Lives and Travels of Mormon and Moroni, pp. 185-186, 189] [See the commentary on Helaman 3:12]