Albert Barnes Commentary 2 Samuel 24:9

"And Joab gave up the sum of the numbering of the people unto the king: and there were in Israel eight hundred thousand valiant men that drew the sword; and the men of Judah were five hundred thousand men." — 2 Samuel 24:9 (ASV)

1 Chronicles 27:23 sufficiently indicates why the numbering was sinful. It is also stated in 1 Chronicles 21:6 that Joab purposely omitted Levi and Benjamin from the count.

Eight hundred thousand ... five hundred thousand - In Chronicles, the numbers are given differently. Therefore, it is probable that the Chronicler has included in his total some numbers that are not included here.