Haven't done them in forever, gained a lot of weight since I used to do them. Im guessing somewhere around 18. When I weighed 50lbs less I could do at least 20.
About 15. Usually I do around 4X8 with a wide grip, keeping reps slow and squeaky clean. One thing I just realized was that by pulling my knees up toward me a bit, I avoid arching my back too much, which helps me with my lower back issues.
I've got a method u could try that I found in Arnold's Encyclopedia. Pick a target # of reps...say 10 for u. Do as many sets as it takes to get those 10 chins...even if it's 10 sets of 1 rep each. Do this 2x a week and before u know it u'll be crankin em out. As u get better at it, increase ur # of target reps. I started on this method 2 months ago when I could only do 5 on the first set. Today it took me 4 sets to get 37 reps. (I'm 225) My buddy is tryin this method and he's noticing improvements in the third week too.