In my 101 Myths of the Bible, I raised questions about whether the sons of Jacob formed the twelve tribes of Israel (Myth 63). The idea wasn’t new or original at the time. Variations on the argument within the scholarly community had existed well before I put my own take into the record. In a recent essay at Thetorah.com, there is a nice academic review of the question: Did Israel Always Have Twelve Tribes? Worth a read.