Greg Wright (personal communication, 16 November 2002) suggests that after Zerahemnah the text is missing the conjoined phrase “and his men”. Later in the sentence, there are two occurrences of the plural pronoun they, both of which refer to Zerahemnah and his men. Perhaps the repetition of men in the proposed original text (“Zerahemnah and his men saw the men of Lehi”) could have led to the loss of the conjoined phrase “and his men”. 𝓞 is almost completely extant here; consequently, we can determine that 𝓞 read without the phrase “and his men”. If the original text had the phrase, then its loss must have occurred as Joseph Smith dictated the text to Oliver Cowdery.
Nonetheless, sometimes the Book of Mormon text, as is common in English, uses the individual name of a military leader to stand for the army as a whole. In fact, sometimes the Book of Mormon text switches between the two possibilities within the same passage (as apparently here in Alma 43:53):
It is obvious in Alma 43:53 that Zerahemnah along with his men saw the men of Lehi and the armies of Moroni on both sides of the river Sidon. The text, like the example in Alma 55:16, assumes that the reader will mentally supply “and his men”.
Summary: Accept in Alma 43:53 the difficult reading that literally refers to only Zerahemnah as seeing that Lehi and Moroni (and their men) have surrounded him (and his men); the reader is expected to recognize that Zerahemnah and his men all saw the same thing.