Just using common averages I could find, a cup of coffee contains 95mg of caffeine. 42 cups is about 3990mg, or 3.9g.
A lethal dose of caffeine is about 10g consumed in a short period of time. 3.9g probably wouldn't kill you and 42 cups would take a while to drink without throwing up. (I personally have consumed quite a bit caffeine when I was a kid and it put me in the ER for a few hours.)
You can try and drink that much coffee, but you won't be a happy camper.
