The Levites were different, not numbered to be in the army, but in the service of the Lord.
Numbers 1 transitions at verses 46 and 47: “All who were numbered were six hundred and three thousand, five hundred and fifty. But the Levites were not numbered among them by their fathers’ tribe.” They were numbered, but in a different way, and not included with the other tribes. Levi was one of Jacob’s sons alright, but his tribe was special. As the Lord explains, “I Myself have taken the Levites from among the children of Israel instead of every firstborn who opens the womb…Therefore the Levites shall be Mine, because all the firstborn are Mine” (Num 3:12-13). Because Levi wasn’t tallied with the men of army age but of God’s firstborn, the Lord instructed, “you shall number every male from a month old and above” (v 15). Apart from Aaron and his sons, Levi was divided into three groups. Their responsibility was to set up, take down, and transport the tabernacle and its furniture: “You shall appoint the Levites over the tabernacle of the Testimony, over all its furnishings, and over all things that belong to it; they shall carry the tabernacle and all its furnishings; they shall attend to it and camp around the tabernacle. And when the tabernacle is to go forward, the Levites shall take it down; and when the tabernacle is to be set up, the Levites shall set it up” (1:50-51). What a prodigious task! When we studied Exodus, we discovered that, apart from the gold, bronze, wood, and linen, the silver alone came to five tons! Chapter 1 concludes with God picturing the camp: Aaron and his sons guarding the front entrance, the Levites protecting the other three sides, and then “everyone by his own camp, everyone by his own standard” (v 52). Yes, all had their own special place, but only because God was in the midst in His own tent, the Heart of the nation.