How many chariots did David kill (2 Samuel 10:18 | 1 Chronicles 19:18)?

Answer:

2 Samuel 10:
17 And when it was told David, he gathered all Israel together, and passed over Jordan, and came to Helam. And the Syrians set themselves in array against David, and fought with him.
18 And the Syrians fled before Israel; and David slew the men of seven hundred chariots of the Syrians, and forty thousand horsemen, and smote Shobach the captain of their host, who died there.

1 Chronicles 19:
17 And it was told David; and he gathered all Israel, and passed over Jordan, and came upon them, and set the battle in array against them. So when David had put the battle in array against the Syrians, they fought with him.
18 But the Syrians fled before Israel; and David slew of the Syrians seven thousand men which fought in chariots, and forty thousand footmen, and killed Shophach the captain of the host.

There are several explanations for these two passages as follows:

1. 2 Samuel 10:17 specifically mentions the battle at Helam, while 1 Chronicles 19:17 does not mention the location. Both of them mention that the Syrians fled before Israel. Therefore, it is possible that 2 Samuel 10 records the specific tally at Helam, while 1 Chronicles 19 records the tally when David’s army pursued the Syrians after they fled, or the overall tally of the battle. In this explanation, any of these options is possible:
   a. 700 chariots and 40,000 horsemen were defeated at Helam. The remaining army fled, and David pursued them, then defeated 6,300 chariots and 40,000 footmen in that pursuit.
   b. 700 chariots and 40,000 horsemen were defeated at Helam. The remaining army fled, and David pursued them then defeated 7,000 chariots and 40,000 footmen in that pursuit. Why did David pursue and slew the remaining army? It’s because when David previously let go of the Syrians when they fled from Israel’s army (2 Samuel 10:13, 1 Chronicles 19:14), they Syrians used that opportunity to gather more forces and attacked Israel again (2 Samuel 10:15-17, 1 Chronicles 19:16-17).
2. The Hebrew word for chariot is רֶכֶב (reḵeḇ). Brown-Driver-Briggs Lexicon defines it as collective chariotry, chariots, a single chariot, riders (collective), or troop (of riders). The word can mean a collective group or a single chariot. Therefore, it may mean that the 7,000 chariots (1 Chronicles 19) were split into 700 chariot units/platoons (2 Samuel 10) each comprising of 10 chariots. 
    
3. Another possibility is from the plain reading of the verses: “the men of seven hundred chariots” = “seven thousand men which fought in chariots”. In other words, the word rekeb means “chariot” in 2 Samuel 10 and means “riders” in 1 Chronicles 19, such that for each chariot (manned by 2-3 riders) to break Israel’s formations, there are 7-8 horse riders following it to engage with Israel’s soldiers and to replace the horse & riders on the chariot if they are killed. Meanwhile, the 40,000 horsemen could either be hybrid soldiers which fight on horse or on foot depending on the battle strategy, or that there were 40,000 horsemen and 40,000 footmen defeated.

These war accounts are historical records. If anyone wanted to falsify or corrupt the Bible, it is unlikely they would falsify these war tallies as it does not affect the religious doctrines at all.
 
Conclusion: No contradictions. David defeated 7,000 people fighting in chariots.