Isaiah 29:5 (King James Bible) and the multitude of the terrible ones shall be as chaff that passeth away
Here in 2 Nephi 26:18, Oliver Cowdery initially wrote multitudes in the printer’s manuscript, but then he immediately corrected multitudes to multitude by erasing the plural s. His correction undoubtedly restored the reading of the original manuscript (which is no longer extant here). The singular multitude is also the reading of the King James Bible.
Summary: Retain the singular multitude, the corrected reading of the printer’s manuscript, which is also the reading of the corresponding Isaiah passage.