Homer, Iliad , XXIV 604, makes them but twelve:⁠—

“Twelve children perished in her halls, six daughters and six blooming sons; these Apollo slew from his silver bow, enraged with Niobe; and those Diana, delighting in arrows, because she had deemed herself equal to the beautifulcheeked Latona. She said that Latona had borne only two, but she herself had borne many; nevertheless those, though but two, exterminated all these.”