Mormon describes many armies consisting of ten thousand in Mormon 6:12-14:
12 And we also beheld the ten thousand of my people who were led by my son Moroni.
13 And behold, the ten thousand of Gidgiddonah had fallen, and he also in the midst.
14 And Lamah had fallen with his ten thousand; and Gilgal had fallen with his ten thousand; and Limhah had fallen with his ten thousand; and Jeneum had fallen with his ten thousand; and Cumenihah, and Moronihah, and Antionum, and Shiblom, and Shem, and Josh, had fallen with their ten thousand each. (emphasis added)
Wouldn’t we expect Joseph Smith to have mentioned more varied numbers if he was writing the story himself? Why would he mention so many armies of 10,000? Interestingly this description is consistent with what we know about Tlascalan armies:
Of the followers of the old Xicotenga . . . there were ten thousand; of another great chief named Moseescaci there were another ten thousand; of a third, who was called Chichimecatecle, there were as many more…– Bernal Diaz del Castillo, The Bernal Diaz Chronicles, trans. and ed. A. Idell (Garden City, New York: Doubleday, 1956), 161–162, 110, 103; cited in John L. Sorenson, An Ancient American Setting for the Book of Mormon (Salt Lake City, Utah : Deseret Book Co. ; Provo, Utah : Foundation for Ancient Research and Mormon Studies, 1996 ), 263
If the Book of Mormon was a product of Joseph Smith, wouldn’t he have been more likely to know of Roman armies of 100 rather than 10,000?