Commentators extensively discuss the different numbers here (800k in Israel, 500k in Judah) versus those in 1 Chronicles (1.1M in Israel, 470k in Judah). They explain this isn't a contradiction but likely due to different counting methods. For example, one count may have included David's standing army of nearly 300,000 men while the other did not. The census was also incomplete, as Joab refused to count the tribes of Levi and Benjamin.