Accepting Mormon’s numbers at face value gives 250,000 dead, including 10,000 from the previous verse, killed on a single terrible day. It seems more probable that this is an exaggerated number and that “ten thousand” names a military unit rather than a specific count. (See Helaman, Part 1: Context, Chapter 4, “The Meaning of Numbers: Counts and Estimates in the Book of Mormon.”) Mormon names twelve captains of ten thousand, meaning that he has omitted the names of ten more such leaders. Perhaps these twelve had a special relationship with him, or perhaps it is a literary image. Symbolically, Israel has fallen, represented by twelve men for the twelve tribes.
Mormon evidently knew that “a few… had escaped into the south countries.” It is not clear how he would have known this information. If they were deserting, they would not have announced their intentions. If they escaped after the battle, they would have taken pains to avoid notice, including by other Nephites. For that matter, it is not clear how the twenty-four survivors found each other.
Although Mormon dismisses the dissenters as “a few,” probably they accounted for the largest number of survivors. Facing obvious defeat and death, many found some way to escape.
This may also be the explanation for the small number of survivors. The remaining twenty-four may have been the last left alive who willingly retained the name of Nephite. Mormon gives us no information about how they survived. The fact that there were two twelves makes the number seem symbolic rather than a count.