In the second year of travel in the desert, Moshe and Aharon were commanded by the Almighty to count all male Israelites between 20 and 60. There were 603,550 available for military service. The tribe of Levi was exempt because of their special duties as religious leaders. (It is probably from here that countries give divinity deferments to clergy and divinity students.)
The twelve tribes were directed regarding the formation (three tribes were on each side of the Portable Sanctuary) in which they were to camp and travel.
The 22,300 Levites were commanded in the Sanctuary service. The family of Gershon was to transport the coverings of the Sanctuary. The family of Kehos carried the Ark, Table, Menorah and Altars. The family of Merari transported the boards, pillars, bolts and sockets. (Aish)