Jamieson, Fausset and Brown on 1 Chronicles 21:5

5. Joab gave the sum of the number of the children of Israel--It amounted to one million one hundred thousand men in Israel, capable of bearing arms, inclusive of the three hundred thousand military (1Ch 27:1-9), which, being already enlisted in the royal service, were not reckoned (2Sa 24:9), and to four hundred seventy thousand men in Judah, omitting thirty thousand which formed an army of observation stationed on the Philistine frontier (2Sa 6:1). So large a population at this early period, considering the limited extent of the country and comparing it with the earlier census (Nu 26:1-65), is a striking proof of the fulfilment of the promise (Ge 15:5).