2 Samuel 8:4
4 And David took from him a thousand chariots, and seven hundred horsemen, and twenty thousand footmen: and David houghed all the chariot horses, but reserved of them for an hundred chariots.
1 Chronicles 18:4
4 And David took from him a thousand chariots, and seven thousand horsemen, and twenty thousand footmen: David also houghed all the chariot horses, but reserved of them an hundred chariots.
So, did David take 700 or 7,000 horsemen?
Firstly, we can recognize that if David took 7,000 horsemen, it's also true that he took 700 horsemen. One of the accounts could simply be being less precise.
However, we see from another alleged contradiction passage that I discussed here that the classification of "horsemen" as opposed "footmen" could be floating or contextual:
2 Samuel 10:18
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 19:18
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.
Therefore, it's likely that the account in Samuel is only counting those specifically trained as horsemen, whereas the account in Chronicles overlaps categories slightly by including the information that certain footmen were also trained as horsemen, so that if needed they could increase their army of horsemen tenfold.
That means there were only 700 that were trained specifically as horsemen, but ten times as many that could technically operate as horsemen among the footmen.
For example, I say:
"There were 1,000 army men, and 50 tank drivers."
Does there need to be 1050 people? Do all 50 tank drivers have to specialize in that role?
No, and in fact, only 10 of them were full-time tank drivers. So I would also be correct in saying:
"There were 1,000 army men, and 10 tank drivers."
It's all going to depend how strict or liberal I am treating the label of "tank driver".
The classification system wasn't set in stone, so the two historians interpret the same data slightly differently.
The writers of Samuel and Chronicles reporting populations in different terms happens frequently, and every time the critics of the Bible declare that it must be a contradiction. Somehow, they are able to authoritatively know the mindsets and perspectives of the writers of these books.
Recognize that two people can look at the same data, and report differently, because they are looking at it from different perspectives, and highlighting what they think is important. We then can get a fuller picture by comparing both accounts, both of which are right.