The 1830 typesetter accidentally omitted the preposition in before multitudes. The meaning of the original text is that these multitudes are a part of the great mother of abominations, not that the great mother of abominations is forming her own armies. The Book of Mormon text includes a good many examples of different peoples gathering together “in multitudes”, especially to fight wars, including these two nearby examples:
For 1 Nephi 14:13, the 1908 RLDS edition, based on the printer’s manuscript, restored the in.
Summary: Restore in the LDS text the preposition in, the reading of the original manuscript, in 1 Nephi 14:13; the Book of Mormon consistently describes people as gathering together “in multitudes” to fight wars.