𝓞 reads “with an army of two thousand men”. When he copied the text from 𝓞 into 𝓟, Oliver Cowdery changed the preposition with to by, probably accidentally. The syntax in this sentence is complex, but the intent of the passage is that the food was sent along “with an army of two thousand men”. Both the food and the extra men were supplements to the larger army. The preposition by is expected, but with certainly works. Interestingly, there is a similar passage involving the verb guard where the original text read by, but that was changed to with:
See under Alma 27:24 for a comparative discussion of these two passages.
Summary: In accord with the reading of the original manuscript, restore the preposition with in Alma 58:8 (“with an army of two thousand men”).