John Gill Commentary Numbers 1:47

"But the Levites after the tribe of their fathers were not numbered among them." — Numbers 1:47 (ASV)

But the Levites
The tribe of Levi were excepted from this muster, they being employed in a kind of warfare, and therefore not to be engaged in another:

after the tribe of their fathers, were not numbered among them :
the rest of the tribes; the reason follows.