Here in the original manuscript, Oliver Cowdery initially skipped the relative pronoun which, thus writing “there were many people went forth”. Later, with somewhat heavier ink flow, Oliver supralinearly inserted the which; his correction could have been made when he read the text back to Joseph Smith. (Later in his editing for the 1837 edition, Joseph emended the which to who, a grammatical change that is fully discussed under which in volume 3.)
Examples of existential there- clauses for which the relative pronoun is lacking can be found elsewhere in the text but virtually always with some postmodifying prepositional phrase for the delayed subject (and possibly other parenthetical elements), as in the following nearby example:
In other words, “there was a large company of men ... departed out of the land of Zarahemla”. Under Enos 1:23, I list additional examples of this kind of construction in the text. But there are also numerous examples of the existential there-clause where the relative pronoun does occur, especially when the delayed subject is short, as nearby in Alma 63:6: “there were many of the Nephites which did enter therein”, not “there were many of the Nephites did enter therein”. For each instance, the critical text will follow the earliest textual evidence, thus which here in Alma 63:9.
Summary: Maintain in Alma 63:9 the corrected reading in 𝓞, the relative pronoun which in the existential there-clause, “there were many people which went forth into the land northward”.