Alma 44:21

Now the number of their dead were not numbered, because of the greatness of the number; yea, the number of their dead were exceeding great, both on the Nephites, and on the Lamanites.

The number of their dead was not counted because there were so many. There were very many dead, both Nephites and Lamanites.

The dead were too many to count. The number was staggering on both sides—Nephites and Lamanites.