The numbering of the people is some form of a census. It is interesting that a census would be taken at all. When Benjamin has a census later in Book of Mormon history, he is dealing with a combined population of Nephites and Zarahemlaites that has had significant defections to the Lamanites. There is excellent reason from a political standpoint to take a census, as it would be difficult to know how many people were actually beholding to the Zarahemla polity.
In the case of the Jaredites, the counting suggests that strongly that a similar mixing and enlargement of population has occurred. If there are only twenty four original adult males and their offspring, we have a limited number of men who must only keep track of their grandchildren (and perhaps an odd great-grandchild). This is not a large number, and it would be rather simple for the twenty four grandfathers to gather and add up their progeny. The very fact of the count tells us that something else has happened. As with Benjamin, circumstances have occurred which make it difficult to assess the numbers of people who are now beholding to the Jaredite polity.
The easiest and most logical way to explain the census is the infusion of “others” who were already living in the area when the Jaredites arrived. As with the Nephites, there was something about the Jaredites that gave them priority or reign, but we should not equate that with cultural dominance. In a very a parallel way to the Lehites, the Jaredites also had to learn the new area, new plants, new planting methods, and new fauna. They were as dependent upon those who had already figured out how to live in the land as were the later Lehites.
Symbolic: The numbers present in the original party of the Jaredites fall into categories that are too neat to be readily accepted as actual counts. We have the number of adult males as 24, or 2x12. Jared had 4 sons, but the total number of children is 12. These are all numbers that are important in either the Biblical heritage or the Mesoamerican heritage (12 from the Old World and 4 in the New). With the only numbers of people showing as such symbolic numbers we should be cautious in assigning specific historical veracity to the them. We can be comfortable in the relatively small nature of the founding Jaredite population, but these particular numbers may tell us more of the subsequent conceptions of numerical symbology among the recorders of the tradition than any real count of the Jaredites.