In the printer’s manuscript, Joseph Smith deleted the plural s in minds in his editing for the 1837 edition, but this change was not implemented in that edition. The 1908 RLDS edition, however, adopted the change to the singular mind since it had been marked in 𝓟. With regard to the 1837 edition itself, perhaps Joseph changed his mind and decided to leave the plural minds. Elsewhere the text consistently refers to “the minds of the people”, never to “the mind of the people”:
In none of these other examples did Joseph ever emend minds to mind. One possible motivation for the singular mind in Alma 17:6 could have been the preceding singular subject and verb forms, this and was (“and also this was the minds of the people”). In other words, perhaps Joseph initially emended minds to mind in Alma 17:6 in order to avoid the clash in number between this was and minds. In any event, the critical text will maintain in Alma 17:6 the use of the plural minds, the earliest reading; such plural usage is found consistently elsewhere in the Book of Mormon.
Summary: Ignore in Alma 17:6 Joseph Smith’s emendation in 𝓟 of the plural minds to the singular mind; the change was never implemented in the 1837 edition, perhaps because elsewhere the text has references to only “the minds of the people”.