How many stalls for horses and chariots did Solomon have? In 2 Chronicles we read:


  ... Solomon had 4,000 stalls for horses and chariots and 12,000 horsemen ...- 2 Chronicles 9:25


... but in 1 Kings:


  Solomon had 40,000 stalls of horses for his chariots, and 12,000 horsemen. - 1 Kings 4:26
Answering how many there actually were isn't going to be possible here because there is a definite conflict between the verses, and the honest answer is "nobody's sure".  And opinion answers don't count here.  Since your question is a valid one asked quite often, I hope you'll indulge me and let me answer something slightly different that still gets at the heart of your question.  

A fair and on-topic question would be "How do Apologists address/resolve this discrepancy?"  I'd like to answer that, since it is answerable.  If that's not what you're looking for, and you really want an answer to how many horses there are, the answer is "we're not sure" and you can stop reading there.  If you want to know how we can read those verses and not scream "The Bible isn't inerrant!!!" read on.

As covered in Rules behind resolving alleged Biblical discrepancies the field of Apologetics hinges on avoidance of logical fallacies in explanations, and sticks to standard Christian doctrines including Biblical inerrancy, infallibility, and inspiration.  

Most Apologetics experts that I'm aware of subscribe to the definition of inerrancy covered at From a Fundamentalist standpoint, what does the phrase “Inspired, infallible, inerrant Word of God” mean?  This is important to define, in order to avoid straw-man arguments.  Short version: The original manuscripts are believed to be inerrant, not the translations. Many translations have had known errors.  copyist errors are well-documented and expected.  Yet, for doctrinal purposes, we have enough manuscripts to know that there aren't any critical doctrinal issues affected by such errors.

With that out of the way, one answer is already obvious.  This could easily be a copyist error.  That's acceptable within the bounds outlined above. The number of horses certainly doesn't affect central doctrines.

The second possibility is that these are two different counts.  Solomon certainly gained enormous wealth during his reign.  The number of horses and horsemen certainly grew, so it it possible that one cont is from early in his reign and another is later in his reign.

Personally, I subscribe to the copyist error, but either way, this discrepancy doesn't pose a problem, either for the doctrine of inerrancy, nor does it have any other theological importance.  

I did some searching, and a very similar error can be found at CARM:

Excerpt: 


  There are two possible explanations for this discrepancy. 1) A copyist
  error. 2) The difference is due to time; that is, one account is at
  the beginning of Solomon's reign (1 Kings 4:26), and the other at the
  end (2 Chron. 9:25). I believe the most probable is a copyist error
  since we can see that Chronicles does have copyist errors in other
  areas. Therefore, it is probable that the same thing occurred here.