Here we have two minor errors based on equivalent meaning. When copying from 𝓞 into 𝓟, Oliver Cowdery initially replaced men with armies. Somewhat later he crossed out armies and supralinearly inserted men, the reading in 𝓞. Oliver probably made this correction when he proofed 𝓟 against 𝓞; not only is the ink flow for the correction somewhat weaker, but the quill is duller. Elsewhere the text has only one other example of “with his men” (in Alma 47:14) but eight of “with his army” and six of “with his armies”, so it is not surprising that Oliver initially replaced men with armies in 𝓟. But any of these readings is possible, so the critical text will follow the earliest reading, in this instance “Teancum with his men”.
The other error of semantic equivalence involves the pronoun they in the relative clause “which they had lost”. The 1906 LDS edition accidentally replaced they with he, perhaps under the influence of the preceding his in the phrase “with his men”. This edition was never used as a copytext, so the secondary he was not perpetuated.
Summary: Follow in Alma 52:15 the reading of the earliest textual sources (including 𝓞) for men and they: “that he might assist Teancum with his men in retaking the cities which they had lost”.