Charles Ellicott Commentary 1 Chronicles 23:3

"And the Levites were numbered from thirty years old and upward: and their number by their polls, man by man, was thirty and eight thousand." — 1 Chronicles 23:3 (ASV)

Now ... and — that is, after the council had agreed upon it.

The Levites were numbered from the age of thirty years and upward. —A census like that which Moses instituted (Numbers 4:3, Numbers 4:23, Numbers 4:30, and so on), of all Levites from thirty years old and upward unto fifty years, for the work of the Tabernacle.

By their polls, man by man. —Literally, As to their skulls, as to men. The second phrase defines the first, and excludes women and children.