The most I have ever done is 55 in one set! I never bother going over 30 now! I stay in the 12-25 range! But I usually add weight to these sets! I have never had a hard time with pull-ups, but I know it can be a bitch for some people!
I can do 26-27 on my first set... but that is all the way up... all teh way down to rock bottom hang.
I took the Marine Corps chinup challenge several years ago. The contest was to see who could do the most chinups. I did 19 all the way up... all the way down... full extension... and they gave the prize to a nitwit kid who did 25 halfway chinups.