The original manuscript has the plural stains, which Oliver Cowdery miscopied as the singular stain when producing the printer’s manuscript. The plural s is especially clear in the original manuscript since it was overwritten (but the preceding n is quite weak).
When the word stain is preceded by a possessive pronoun (like our), the Book of Mormon text uses the plural stains; there are two more instances of our stains, both occurring nearby:
The plural stains is extant in 𝓞 for the first of these two other examples. The original use of the plural stains in Alma 24:11 is undoubtedly correct and will be restored in the critical text. Of course, the singular stain is also possible, as in the text for Alma 5:21: “yea his garments must be purified until it is cleansed from all stain”.
Summary: Restore in Alma 24:11 our stains, the reading of the original manuscript; the plural our stains also occurs nearby in Alma 24:12 and Alma 24:15.