Answer:2 Samuel 8:4 -- 1,700 horsemen
1 Chronicles 18:4 -- 7,000 horsemen
Are these two passages related with each other? Why the discrepancy?
The two passages do deal with the same event, but they are worded differently. The following is taken from the New King James Version:
"David took from him one thousand chariots, seven hundred horsemen, and twenty thousand foot soldiers. Also David hamstrung all the chariot horses, except that he spared enough of them for one hundred chariots" (II Samuel 8:4).
"David took from him one thousand chariots, seven thousand horsemen, and twenty thousand foot soldiers. And David also hamstrung all the chariot horses, except that he spared enough of them for one hundred chariots" (I Chronicles 18:4).
The three words marked boldly in red were added by the translators. They do not appear in the original text. When you compare the two verses, it becomes apparent that the translators noticed the difference in the readings and added a few words in an attempt to bring them closer to saying the same thing.
The translators of the New American Standard Version took a different interpretation:
"David captured from him 1,700 horsemen and 20,000 foot soldiers; and David hamstrung the chariot horses, but reserved enough of them for 100 chariots" (II Samuel 8:4).
"David took from him 1,000 chariots and 7,000 horsemen and 20,000 foot soldiers, and David hamstrung all the chariot horses, but reserved enough of them for 100 chariots" (I Chronicles 18:4).
Instead of inserting an assumed "chariots" between the thousand and the seven hundred in II Samuel 8:4, they read the text as saying thousand plus seven hundred horsemen.
The English Standard Version notes that the Septuagint translation, done before the time of Jesus, has seven thousand in II Samuel 8:4 instead of seventeen-hundred. The 1,700 comes from the Masoretic Text.
The most common explanation is that the two passages once read the same, but some scribe made a mistake in a copy causing the two readings to diverge. I have a hard time accepting this explanation as the difference between the two texts involve several words. The difference cannot be explained as an accidental stroke difference or the adding, subtracting, or changing of a letter. The argument almost entirely resides on the Septuagint translation.
One suggests there were two different battles. In II Samuel 8:3 it says, "David also defeated Hadadezer the son of Rehob, king of Zobah, as he went to recover his territory at the River Euphrates." But in I Chronicles 18:3 it says, "And David defeated Hadadezer king of Zobah as far as Hamath, as he went to establish his power by the River Euphrates." Personally, I think this argument is very weak because the list of what was destroyed and captured are too similar.
Difference in What Was Counted
Since the two verses are worded differently, another explanation is that I Chronicles 18:4 is counting the number of men involved while II Samuel 8:4 was counting the number of chariot companies, where it is assumed that there were ten men per company. This explanation was promoted by Matthew Henry and John Wesley.
A similar, but even better explanation is proposed by Peter Ruckman based on II Samuel 10:18 and I Chronicles 19:18.
"Then the Syrians fled before Israel; and David killed seven hundred charioteers and forty thousand horsemen of the Syrians, and struck Shobach the commander of their army, who died there" (II Samuel 10:18).
"Then the Syrians fled before Israel; and David killed seven thousand charioteers and forty thousand foot soldiers of the Syrians, and killed Shophach the commander of the army" (I Chronicles 19:18).
By the way, this is an account of another battle between David and Hadadezer, king of Syria. The obvious explanation is that Syria had multiple men ridding in each chariot (ten to be exact). This allowed one to concentrate on driving while others fought on each side of the chariot, plus a commander and backup in case a man was killed. One source lists the following in a typical Syrian chariot crew: a commander, drivers, shieldmen, and bowmen. Therefore, one account is saying that David killed the men in 700 chariots while the other is saying David kill 7000 men who rode in the chariots. This then also explains how II Samuel 8:4 can mention 1000 chariots, but only 700 horsemen. If the 700 horsemen were units, then we have men to ride in 700 chariots plus 300 spare chariots in case some were damaged in battle.
I believe this last explanation is the most natural fit.