Both among and amongst are found in the Book of Mormon text, but by far the more frequent one is the modern English among, with 582 occurrences in the original text. Still, there are 14 occurrences of the archaic amongst. Here in 𝓟, Oliver Cowdery initially wrote the more expected form among, but then almost immediately he corrected the word to amongst by inserting inline the st, with the t slightly raised (there is no change in the level of ink ﬂow for the entire correcting st). Most probably, the original manuscript had the archaic amongst here in Alma 20:13. This same initial error is found a little later in the text:
In this instance, the correction in 𝓟 was made by the unknown scribe 2, who seems to have been proofing 𝓟 against 𝓞 for that part of the text (see the discussion under Alma 22:27).
Summary: Accept in Alma 20:13 Oliver Cowdery’s virtually immediate correction of the expected among to the more difficult archaic form amongst.