The printer’s manuscript has the singular church, the 1830 edition the plural churches. The plural is very likely correct. Here the text is speaking of leaders of churches (and their teachers) who will compete with each other in trying to get members. A given leader or teacher is envious of those who belong to other churches. Here churches acts as a distributed plural and is consistent with the overall plurality in this passage. Normally, however, people belong to “a church”. Elsewhere the Book of Mormon has 27 references to people belonging to a church, but never to people belonging to churches. It would appear that the expected singular led scribe 2 of 𝓟 to write the singular here in Mormon 8:28 instead of the correct plural. For another case where an unexpected plural churches was replaced with the singular church, see under Alma 8:23.
Summary: Maintain the plural reading churches in Mormon 8:28; the singular church in the printer’s manuscript is most probably an error made by scribe 2 of 𝓟.