The 1837 edition replaced exceeding with much here in Alma 58:2. This change is probably due to Joseph Smith’s editing, although the change is not marked in the printer’s manuscript. If exceeding had been retained in the text, it would have been edited to exceedingly in the 1981 LDS edition (if not earlier in the 1920 LDS edition). See the discussion under 1 Nephi 2:16 as well as more generally under exceeding in volume 3.
The phrase “so exceeding more ” is unique in the Book of Mormon text. However, there are examples in the text of “exceeding more numerous” without the initial so:
There is also an example in the earliest text where the word order is reversed: “they saw that the Lamanites were more exceeding numerous than they” (Helaman 4:25); for discussion of the word order in that sentence, see under that passage. In addition, there are examples in the text of “so exceeding ”:
The only difference in Alma 58:2 is that the adjective takes the comparative form, more numerous.
Thus the original reading, “so exceeding more numerous”, will be restored in Alma 58:2.
Summary: Restore in Alma 58:2 the original exceeding in place of the secondary much (thus “they were so exceeding more numerous than was our army”).