Oliver Cowdery initially wrote the singular priest here in 𝓟, but of course king Noah had many priests (Mosiah 11:4–5). Almost immediately Oliver inserted the plural s inline; there is no change in the level of ink flow.
Summary: Maintain in Mosiah 11:19 the plural priests (the corrected reading in 𝓟); the plural usage agrees with every other reference to the priests of king Noah.