דברי הימים א, פרק כ״ג, פסוק ג׳

A census of the Levites was conducted to organize the holy service and identify those fully prepared to take on their sacred duties. The counting focused exclusively on individuals who were thirty years of age and older. This specific age requirement was chosen because it marks the official time when a Levite begins his active service, following the original guidelines set down in the Torah [מלבי״ם, ביאור שטיינזלץ].

To ensure an exact tally, the counting was done on an individual basis. Rather than a general estimate, it was a precise, head-by-head census of the adult males [מצודת ציון]. Some explain that the men were not counted directly by pointing to each person; instead, the census was conducted by tallying heads, mirroring the way the Israelites were counted earlier in the Book of Numbers [רלב״ג, ביאור שטיינזלץ]. When the careful counting was complete, the final number of men ready for service reached thirty-eight thousand.