I drank 16 in one go (yeah...erp!) and it only counted once towards my twelve - quest should probably read that shot-gunning is not recommended rather than optional ;)
Edit - OK it's a bit more complicated than that - so I tried to drink 16, but was only able to drink one, so I guess what I was expecting was a message to tell me that I could only drink one at a time, or that I was too buzzed to drink any more than that in one go (which did occur once I'd had three). Not really a bug, more of a little tweak for clarity perhaps?