Jamieson, Fausset and Brown on 1 Chronicles 18:4-8

4-8. And David took from him a thousand chariots--(See on 2Sa 8:3-14). In 2Sa 8:4 David is said to have taken seven hundred horsemen, whereas here it is said that he took seven thousand. This great discrepancy in the text of the two narratives seems to have originated with a transcriber in confounding the two Hebrew letters which indicate the numbers, and in neglecting to mark or obscure the points over one of them. We have no means of ascertaining whether seven hundred or seven thousand be the more correct. Probably the former should be adopted [Davidson's HERMENUTICS].

but reserved of them an hundred chariots--probably to grace a triumphal procession on his return to Jerusalem, and after using them in that way, destroy them like the rest.