How many Horsemen did David Capture,
1700 or 7000? 
2 Samuel 8:4 and 1 Chronicles 18:4
 

1. 1,700 Horsemen (2 Samuel 8:4) - "And 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
2. 7,000 Horsemen  (1 Chronicles 18:4) - "And 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."

     This is most probably a copyist error or a copy of a manuscript where the number was altered through decay.  1 Chron. 18:4 is probably the correct number since it is a better preserved manuscript.
     "Probably on earlier copyist inadvertently omitted the word "chariot" that we find supplied in some translations.  This in turn created a problem for a later copyist who would have recognized that it was not proper Hebrew structure to write "one thousand seven thousand" horsemen," so he would have reduced the second "thousand" to "hundred" resulting in the reading we now have in second Samuel 8:4."¹

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1. When Critics Ask. p. 171-172.