Here the 1920 LDS edition changed the preposition to to at. The resulting reading is what modern English readers expect. Nonetheless, other examples of this same usage with to occur in the original text, many of which have never been edited. Consider, for instance, the following three cases where the verb phrase “to gather together” retains the preposition to:
For other examples of where the preposition to has been edited to at, see the discussion under 1 Nephi 17:14.
Summary: Restore the original preposition to in Mosiah 18:7 (“there were a goodly number gathered together to the place of Mormon”); similar examples have been retained in the text.