If we are to accept Mormon’s numbers at face value, we have 250,000 (including the 10,000 from the previous verse) who were slaughtered on that terrible day. While this might not be out of the question, there is every probability that this is an exaggerated number due to the nature of the “ten thousand” number as a probable marker of military units rather than specific counts (see the essay on counts in the Book of Mormon for more information).
In addition to the possibility that the numbers are generalized and not intended to represent real casualties, we have the interesting way that Mormon presents the information. There are twelve specifically named men who led their “ten thousand”. Since there were ten more, we have the interesting case of Mormon naming most, but not all. It might be that like Moroni, he was closer to those named men. However, the very fact that there are twelve of them seems to beg to be seen symbolically. Mormon is telling us that the remnant of Israel has fallen, represented by twelve men for the twelve tribes.