Convert fresh basil leaves to a dry measurement
This is a more tricky conversion than most.  As another answer already said, the "standard" conversion for most herbs is 3 parts fresh = 1 part dried.  (There is more general advice on that question in the link rumtscho gave in comments here.)

Basil is a particular problem because its flavor is generally very different in dried vs. fresh forms.  Dried basil can also vary greatly in its potency and flavor notes: I often find it has enhanced "sweetness" compared to fresh basil, but lacks some of the other distinctive flavors of the fresh form.  The only time I'd generally consider substituting dried basil in a recipe for fresh is if the basil is cooking in the dish for a long time (for example, simmering an hour or more).

In any case, sometimes you just can't get the fresh stuff.  But converting a number of leaves into a measurement of dry basil gets even trickier: what size is a "standard" basil leaf?  Is your particular dried basil fresh-bought and potent, or has it been sitting around for a while?

I poked around in internet searches, and people seem to recommend anywhere from 4 to 8 fresh basil leaves = 1 teaspoon of dried basil.  So, if I had to do this, I'd go somewhere in the middle and probably try 2 teaspoons of dried basil for 12 leaves.  If you are using this in a dish that will cook for a while you'll have time to adjust, so I'd start a little on the lower side and add more basil to taste if necessary.

On the other hand, if you're using this in a dish where you're depending on a specific "fresh herb" flavor of fresh basil, there's just no substituting the dry form.  In that case, I might recommend trying another fresh herb you do have (perhaps something more neutral like parsley) mixed with a little dried basil to give a bit of the right flavors.  It still won't be the same as fresh basil, but at least you'll get a bit of the fresh herbiness which is often part of the experience too.