First off, have you seen your doctor about this? This could be something innocuous like low blood sugar or not being in the best shape to start with. It could also be something considerably more serious. And is it specifically the crunches that cause the dizziness? Do you get the same dizziness if you do a standing crunch (standing upright, hands behind your head, and curling down as if you were doing a crunch on the floor)? Are you breathing throughout the crunch or are you holding your breath perhaps?
If you haven't done a lot of abdominal exercise, honestly, it might simply be that you're not in very good shape. Crunches are harder than people give them credit for. If you can't see a doctor, I recommend cutting your number of crunches a little lower for a week or so, say only do 5, or do them from a standing position (assuming those don't make you dizzy). It may feel like you're not getting much exercise, but you're teaching your body to accept it. And make sure that you're breathing as you do it. Don't hold your breath.
Lastly, it's possible that you're dealing with Benign Paroxysmal Positional Vertigo, a disorder that happens from excessive deposits in the eardrum, although that commonly manifests itself with any positioning where your head is not upright, so you probably would be running into it in other situations. If it is BPPV, one option is to switch to other exercises. Planks are recommended in the link above. Getting up to holding it for 60 seconds (and again, don't stop breathing during it) takes most people a bit. Another is treatment by a doctor, and we're back to my initial point. It's better to be safe than sorry.