Here we have one more example of singular thou used for plural subjects. Later on in this chapter, there are five more instances of such usage (see the discussion under Mosiah 12:30–31). In his editing for the 1837 edition, Joseph Smith replaced all six of these cases of singular thou or thine with the plural. The critical text, on the other hand, will restore all these cases of the second person singular pronoun. For previous discussion of this usage, see under 1 Nephi 3:29; for a complete discussion, see thou in volume 3.
Summary: Maintain the occasional use of thou for plural subjects whenever it is supported by the earliest textual sources; such usage is clearly intended since it occurs fairly frequently in the text (especially here in Mosiah 12, when Abinadi is speaking to the priests of king Noah).