Here in the original manuscript, Oliver Cowdery initially wrote the singular synagogue (the initial s was capitalized in 𝓞); then virtually immediately Oliver supplied the plural s (there is no change in the level of ink flow for the s that was inserted inline). The surrounding text consistently shows that there was more than one synagogue among the Zoramites:
For another example showing Oliver’s tendency to omit the plural s for synagogues, see under Alma 23:2.
Summary: Maintain in Alma 32:10 the plural synagogues, the corrected reading in 𝓞.