I'm confused, when David smote also Hadadezer, king of Zobah, how many horsemen did he take?

Good question. These books where not complied into one "old testament" until centuries after they were originally written separately. In that time is is very possible that small miscopies can occur. When you think about it, it is remarkable that they are even that similar! There are such minor inconsistencies in the Bible, which I why I prefer not to emphasize its "infallibility" as much as its "authoritativeness", that is, its ability to teach us the principles of God, his character, will, and how he interacts with those he's created, and it give us trustworthy direction on how to live our lives.