Here Oliver Cowdery initially wrote the plural mights, then immediately erased the plural s. In the original text, there are instances in plural contexts of both the singular might ( 7 occurrences) and the plural mights (12 occurrences). Except for one example in Jacob 5:72, all the instances of mights have been edited in the LDS text to the singular might. Here in Mosiah 10:19, we apparently have one of the original singular cases. The original manuscript is not extant here, but Oliver Cowdery’s immediate correction (by erasure) in 𝓟 implies that 𝓞 read in the singular. A similar example showing Oliver’s tendency to write mights instead of might is found near the end of the book of Alma:
In that instance, 𝓞 is extant and reads in the singular. Here in Mosiah 10:19, Oliver appears to have made the same kind of initial error in 𝓟. For a complete discussion of might(s), plus a list of all the cases, see under Jacob 1:19.
Summary: Retain in Mosiah 10:19 the corrected reading in 𝓟, the singular might (“I did stimulate them to go to battle with their might”); Oliver Cowdery’s correction is immediate, which implies that 𝓞 also read in the singular.