Only thou shalt not number the tribe of Levi, neither take the sum of them among the children of Israel:

Wesley's Notes

1:49 Levi - Because they were not generally to go out to war, which was the thing principally eyed in this muster, Nu 1:3,20,45, but were to attend upon the service of the tabernacle. They that minister upon holy things, should not entangle themselves in secular affairs. The ministry itself is work enough for a whole man, and all little enough to be employed in it.

Matthew Henry's Concise Commentary

1:47-54 Care is here taken to distinguish the tribe of Levi, which, in the matter of the golden calf, had distinguished itself. Singular services shall be recompensed by singular honours. It was to the honour of the Levites, that to them was committed the care of the tabernacle and its treasures, in their camps and in their marches. It was for the honour of the holy things that none should see them, or touch them, but those who were called of God to the service. We all are unfit and unworthy to have fellowship with God, till called by his grace into the fellowship of his Son Jesus Christ our Lord; and so, being the spiritual seed of that great High Priest, we are made priests to our God. Great care must be taken to prevent sin, for preventing sin is preventing wrath. Being a holy tribe, they were not reckoned among other Israelites. They that minister about holy things, should neither entangle themselves, nor be entangled, in worldly affairs. And let every believer seek to do what the Lord has commanded.