I believe the stock sprockets on '95 and newer KDX's are 13 front and 47 rear.
Depending on where and how you ride, it may be beneficial to change the
gearing to suit your needs. Most guys using the KDX in tight, technical
stuff will add a tooth or more in the rear (from 47 to 48-50). This lowers
the gearing, meaning that you can run a higher gear (like 2nd or 3rd) in
tight terrain for less wheelspin. On the opposite end of the spectrum, if
you were riding open desert type stuff, a smaller sprocket in the rear would
give you considerably more speed if you spent a lot of time in 5th or 6th
gear.
Also note that changing the number of teeth on the front sprocket has the
opposite effect that is does on the front, and a one tooth change on the
front is roughly equal to a three tooth change on the rear. Meaning 12/47
yields a similar effect to 13/50.
In my opinion, RIDE THE BIKE STOCK FIRST before you start making changes
based on someone else's suggestions. You might like the gearing the way it
is for where and how you ride. Besides, the stock chain is garbage and will
stretch on every ride. By the time you're ready for a new chain (get an
o-ring) and sprockets (replace them together with the chain), you'll have
a better idea what YOU need gearing wise.