It's impossible to compare considering all the cheating going on with chins. I see guys doing what's called kip ups where just jerk your entire body up to create upward momentum calling it a pullup. Others cheat by not going all the way up and not going to a dead hang.
I'll answer like this. 17 real pullups with an over hand grip. With an under hand 20. Doing kip ups and not getting a full range of motion way over 20.