Here we see some variation in number for the words flocks and pasture. As discussed earlier under Alma 17:27, the plural flocks is correct. The change to the singular flock in the 1906 LDS edition was most likely a typo.
In his copying from 𝓞 into 𝓟, Oliver Cowdery initially wrote the plural form pastures; then virtually immediately he corrected pastures to the singular pasture (there appears to be no change in the level of ink flow for the crossout of the plural s at the end of the word). Since either reading is possible here, there would have been no motivation for Oliver to emend pastures to pasture; the original manuscript most probably read pasture. There are only two other occurrences in the text of pasture(s), neither of which shows any variation in number. The first, in 1 Nephi 21:9, is an Isaiah quote that has the plural pastures; the second one, in 1 Nephi 22:25, uses the idiomatic expression “to find pasture”, which is also found in John 10:9: “he shall be saved and shall go in and out and find pasture”. Here in Alma 17:39 we must therefore rely on the earliest textual sources rather than on internal linguistic evidence. The best solution is to accept Oliver Cowdery’s virtually immediate correction to the probable reading of the original manuscript.
Summary: Maintain the plural flocks in Alma 17:39, the reading of the earliest textual sources; also accept Oliver Cowdery’s singular pasture, his corrected reading in 𝓟.