I've talked to a lot of people here in the AA/NA program and most of the people that go to meetings all through out the day drink at least 3-4 cups per meeting and go to at least 4 meetings a day. They tell me that they drink coffee at every meetings, and hit up Starbucks after the meetings. In general I believe that they drink upward to 15 or so cups of coffee, and if you want to compare espresso in the mix then it would be upward in the 20's.
I down coffee all through out the day, Hell I even drink about 2 cups at a 8pm AA meeting.
My self personally I drink about 8 cups a day. (They are those little Styrofoam cups).
In my opinion I have a suspicion that coffee is harmless until you begin to notice hyperactivity moments. Things such as nervousness, twitching, insomnia, and much more. Almost the same symptoms as cocaine produces.