John Gill Commentary Numbers 3:40

"And Jehovah said unto Moses, Number all the first-born males of the children of Israel from a month old and upward, and take the number of their names." — Numbers 3:40 (ASV)

And the Lord said to Moses
After he had taken the number of the Levites:

number all the firstborn of the children of Israel, from a month old
and upward, and take the number of their names ;
that they might be compared with the number of the Levites, and the difference between them observed.