Red Bull spokesperson recommended the consumption of no more than one to two cans.
There are examples of people suffering heart disturbances after drinking eight cans of Red Bull.
From
http://en.wikipedia.org/wiki/Red_Bull
this also might be of interest. (1 can red bull, approx one coffee in caffeine.
http://news.bbc.co.uk/1/hi/england/wear/6944026.stm