As discussed under Alma 56:10, Oliver Cowdery accidentally replaced enormity with the impossible enumerority here in Alma 57:13. The original manuscript is extant here, and it reads enormity.
In this verse, the original manuscript clearly has the singular number, but Oliver Cowdery miscopied it as numbers in the printer’s manuscript. Elsewhere in the text, there is one other occurrence of “enormity of one’s number(s)”, and this example also has number rather than numbers: “and also seeing the enormity of their number / Teancum thought it was not expedient that he should attempt to attackt them in their forts” (Alma 52:5). More generally, either number or numbers is possible; for discussion of this point, see under Alma 30:2.
Summary: In accord with the reading of the original manuscript, restore the singular number in Alma 57:13 (“notwithstanding the enormity of our number”); the original enormity will also be maintained rather than the problematic enumerority that Oliver Cowdery initially wrote in 𝓟.