Jamieson, Fausset and Brown on Joshua 14:4

4. The children of Joseph were two tribes, Manasseh and Ephraim--As two and a half tribes were settled on the east Jordan, and the Levites had no inheritance assigned them in land, there would have been only eight and a half tribes to provide for. But Ephraim and Manasseh, the two sons of Joseph, had been constituted two tribes (Ge 48:5), and although Levi was excluded, the original number of the tribes of Israel was still preserved.