In his editing for the 1837 edition, Joseph Smith removed the subject-verb disagreement in this passage (“there are not one among you”) by replacing the singular one with the word any (which permits the plural are). An alternative grammatical emendation would have been to replace the are with is (“there is not one among you”).
This is the only example where one has been changed to any (or vice versa) in the history of the text. Elsewhere, the text has examples of both one among and any among, including the following cases where either one or any will work:
Each of these instances has an existential there, and in three out of the four cases the delayed subject is one, which argues that the use of one in Mosiah 2:34 is expected (although not required). The less intrusive grammatical emendation for Mosiah 2:34 would therefore be to replace are with is. The critical text will, of course, retain the nonstandard usage, “there are not one among you”. For additional discussion of such issues regarding number, see subject-verb agreement in volume 3.
Summary: Retain the original case of subject-verb disagreement in Mosiah 2:34: “there are not one among you” (the earliest extant reading).