Numbers 17:6

Besides, &c. Hebrew and the other versions, "the rod of Aaron was in the midst of their rods." Whence some infer, that there were only 12 rods. But Origen, (hom. 9,) and most others, allow 13, as the tribe of Joseph was divided into those of Ephraim and Manasses; (Calmet) each of whom had a proper representative or prince. See chap. ii. 18, 20; x. 22; xiii. 9, 12. The tribe of Levi is generally placed by itself. If there were only 12 rods, whether would the name of Elisama, or that of Gamaliel, designate the tribe of Joseph? Who was properly the prince of that udivided tribe? (Haydock)