Here Oliver Cowdery’s eye skipped from priests to priest as he copied this passage from 𝓞 into 𝓟. The error was only momentary since Oliver virtually immediately corrected 𝓟 by supralinearly inserting “even one priest” (there is no change in the level of ink flow). Clearly, the passage would not have made much sense without the phrase.
Summary: Maintain in Mosiah 18:18 the corrected reading in 𝓟: “Alma … ordained priests / even one priest to every fifty of their number did he ordain”.