The original manuscript is not extant here; spacing between extant fragments cannot indicate whether the plural s was there or not (although in the transcript of 𝓞 for this passage I conjectured that 𝓞 read in the plural). Oliver Cowdery wrote the singular provision in the printer’s manuscript, but the 1830 compositor set the plural, which is what we expect in current English. Elsewhere there are three occurrences of “a supply of provisions” (that is, with the plural provisions) but none with the singular:
The Book of Mormon has 33 more occurrences of provisions but only one more of provision:
𝓞 is not extant for much provision, but both the 1830 edition and 𝓟 (which in 3 Nephi are firsthand copies of 𝓞) have the singular provision, so 𝓞 probably read as such. (The original text allowed much to modify plural nouns; for discussion of this point, see under Enos 1:21.) Elsewhere we have two occurrences of much provisions and two of many provisions:
These last two examples suggest the possibility that the singular much provision in 3 Nephi 4:18 is an error (originating in 𝓞) for much provisions. This emendation would mean that the original Book of Mormon text contained no other examples of the singular provision, which suggests that the one example of “supply of provision” in 𝓟 for Alma 57:17 also originally read “supply of provisions”. This result would be in complete contrast with the King James Bible, which has examples of only the singular provision (11 times).
Ultimately, there is nothing wrong with the singular provision here in Alma 57:17 (“a new supply of provision”) or in 3 Nephi 4:18 (“because of their much provision”). For each case of provision(s), the critical text will follow the reading of the earliest textual sources, even in those cases where modern English speakers expect the plural.
Summary: Restore in Alma 57:17 the singular provision in “a new supply of provision”, the reading of the earliest extant source (𝓟); similarly, the text will maintain the singular provision in 3 Nephi 4:18 (“because of their much provision”), again the reading of the earliest textual sources (𝓟 and the 1830 edition).