Those look exceptionally bad.
I would say 600 is a safe bet... but you may want to start as 400 and work your way up to 2000-3000 grit making sure you have completely removed all of the sanding marks from the previous step before moving on to the next one.
Then compound and polish.
Follow the steps outlined in this article...
Restore Your Heavily Oxidized Headlights by Kevin George
Post some before/after pictures!!!!