The question comes to us from T. who asks, “How many fighting men were found in Judah?”
9 Joab reported to the king the number of those who had been recorded: in Israel there were eight hundred thousand soldiers able to draw the sword, and those of Judah were five hundred thousand.2 Samuel 24:9
5 Joab gave the total count of the people to David. In all Israel there were one million one hundred thousand men who drew the sword, and in Judah four hundred seventy thousand who drew the sword.1 Chronicles 21:5
Both of these passages are talking about the same event. However they are describing the event in different ways. The difference is in what is being counted. 1 Chronicles says all Israel, but 2 Samuel just says Israel. So what's the difference? How do we know this matters?
First, the difference of 300,000 is alluded to by the size of the active field army. In 1 Chronicles 27:1, we learn that the standing army consisted of 12 divisions of 24,000 men. That comes to 288,000 men. Later in 1 Chronicles we learn that there were officers for each tribe's leader. The number isn't given, but if we assume that these units had about 1,000 men, then the total standing militia was very close to 300,000 men. Therefore, the most logical conclusion is that 2 Samuel 24 didn't include the already active military, while 1 Chronicles 25 added them in because the number included all in Israel.
Second, the difference between Judah's two numbers is 30,000. We are told again that there were 30,000 men assigned to patrol the Philistine boarder in 2 Samuel 6:1. In further support of this, 1 Chronicles 21:5 doesn't say all of Judah like it did with Israel. This shows that the active (in-the-field) military was excluded from the count when “Judah” or “Israel” was being discussed, but these people were included in the count when “All Judah” or “All Israel” was being discussed.
1 Chronicles 21:6, the very next verse, states “6 But he did not include Levi and Benjamin in the numbering, for the king’s command was abhorrent to Joab.” Thus it is likely that the smaller number in Chronicles did not include fighting members of Levi and Benjamin.
Another possibility is to remember that a plague came as a result of this numbering as stated in 2 Samuel 24:15. The plague killed 70,000 men, and the case could be made that the differeing accounts would have represented pre and post-plague numbers.
For Judah's number, it is likely the author of 2 Samuel 24:5 simply rounded his numbers to the nearest 100,000 and 470,000 rounded is 500,000. So 2 Samuel would be reporting to the nearest 100,000, but 1 Chronicles would be reporting a more exact number, with a second digit of significance; 1.1 times a million and 4.7 times a hundred thousand (1.1 and 4.7) versus 8.0 and 5.0 times one hundred thousand. This seems to make logical sense, especially since the idea of there being 470,000 versus 500,000 men is not a terribly important discussion.
While we can't say with absolute certainty which reason is correct for the two sets of numbers, the fact that there are several reasonable explanations means there is no need to conclude that there is a conflict between the two records.