Isaiah 52:8 (King James Bible) thy watchmen shall lift up the voice
In this quote from Isaiah, the earliest text has the plural watchmen, which agrees with the plural in the King James Bible for Isaiah 52:8. The 1879 LDS edition accidentally replaced the plural with the singular watchman, which continued in the LDS text until the 1981 edition was corrected to the plural in 1983. For discussion of a similar mix-up with the word workmen, see under Mosiah 11:10.
The plural is supported by the original Hebrew as well as by the King James use of they later on in this verse:
The plural watchmen is also found everywhere else in the Book of Mormon when this same passage is again quoted (although paraphrastically in the last case):
See under Mosiah 15:29 for discussion of the variant “their voice”.
Summary: Maintain the plural watchmen in Mosiah 12:22; the entire passage supports the use of the plural.