John Gill Commentary 1 Chronicles 9:22

"All these that were chosen to be porters in the thresholds were two hundred and twelve. These were reckoned by genealogy in their villages, whom David and Samuel the seer did ordain in their office of trust." — 1 Chronicles 9:22 (ASV)

All those that were chosen to be porters in the gates were
two hundred and twelve
As fixed in the days of David, and might not be fewer:

these were reckoned by their genealogies in their villages ;
where they dwelt:

whom David and Samuel the seer did ordain in their set office ;
the scheme was first drawn by Samuel the prophet, and communicated to David, who put it into execution, to be constantly and perpetually observed.