How many horsemen did David take? I don’t know. If I had to choose, I’d go with 7,000, since that’s what the LXX records. Most versions have this discrepancy, but not LXX.
We’ve noted from time to time that copyist errors do occasionally enter the text. Such are rare, but do exist. More times than not, they deal with numbers or with names. The significance of such errors are overstated by the opponents of the Bible. If the same were to have happened in the writing of Plato, Aristotle, or any other writing from antiquity, it would be a minor footnote, if mentioned at all.
The books of the Bible in their original transmission were divinely inspired and without error. The scribes who made copies of these were not inspired, nor the scribes who made copies from those copies, etc.. Equally, translators (for the Bible was originally given in Hebrew, Aramaic & Greek) were not inspired. So how can we have confidence in the Bible? It’s a translation of a copy of a copy of a copy… The vast copying process is actually what gives us confidence. There are so many ancient copies available—they serve to confirm the reliability of the word, and show how rare and how minor copyist errors are.