In 1 Nephi 3:31 we read:
And after the angel had departed, Laman and Lemuel again began to murmur, saying: How is it possible that the Lord will deliver Laban into our hands? Behold, he is a mighty man, and he can command fifty, yea, even he can slay fifty; then why not us?
Did Laman really have exactly 50 men? Michael R. Ash explains the significance of this number:
To modern readers this sounds like a small army indeed, but to those of the ancient Near East, the size of Laban’s garrison fits neatly into Old World customs. According to Dr. Hugh Nibley, a permanent garrison in a big city of Lehi’s day consisted of thirty to eighty men. In a recently discovered letter of Nebuchadnezzar (a contemporary of Lehi,) the king speaks of a garrison of “fifty”. In Babylonia, a platoon in the army consisted of fifty men. This permanent unit was always called a “fifty” just as Nephi spoke of “Laban with his fifty”.– Michael R. Ash – Faith and Reason 22: Laban and his “Fifty”
How would Joseph Smith know about garrisons of fifty? Where would he get this number from?