Can do about 15, however chins are done notoriously badly - how many times do you see people rocking, using momentum and not extending their arms on the negative.
I do them at the end of my back workout, but as singles.
I pull quickly, then do a super slow negative (about 10 seconds). Superset this with weighted plate crunches (as a V up) until you get 20
Ouch ,your arms kill for days.
-I call them "Hospitals" in my training diary.