Honestly depends what you will be running over, but a good all around number is a couple of pounds under what is call maximum on listed on the tire. I run 10-11 in my EVO's but they are stiff as hell
I agree with Joey here. Ran 15# in stock deluxe wheel package from RRB to Windrock and it was too low. Got major scares! Ran 20# on the way back felt much better.