The 1953 RLDS edition here replaced the plural preparations with the singular preparation, perhaps unintentionally since elsewhere the 1953 edition left unchanged the number for the word preparation(s). In most instances, the earliest text has the plural preparations, but in a few instances the earliest text reads in the singular. For the specific phrase “to make preparation(s) for war”, the earliest sources all read in the plural (eight times); see the list under Jarom 1:8. This same plural usage is found in examples where the text reads “to make preparation(s)” without any postmodifying “for war” (nine times):
Thus with the verb make we always get preparations (17 times in all). In the remaining cases (when the verb for preparation(s) is not make), the usage between singular and plural is evenly mixed (with three in the singular and three in the plural):
Of course, in the case of Alma 32:6, the singular preparation is required since it is preceded by the indefinite article a.
In all, there are 23 instances of preparation(s) in the text, of which six show textual variation in number: Jarom 1:8, Mosiah 10:7, Alma 24:4, Alma 49:9, Mormon 2:4, and Mormon 4:6. In each case, the critical text will follow the earliest textual evidence in determining whether preparation should be in the singular or plural.
Summary: Maintain the plural preparations in Mosiah 10:7, the reading of the earliest textual sources (“that I might discover their preparations”).