From Weiten – Pyschology Themes and Variations – 7th Edition 2007.
 
Experts believe that there are roughly 100 billion neurons in the human brain. Obviously, this is only an estimate. If you counted them nonstop at the rate of one per second, you’d be counting for over 3000 years! And, remember, most neurons have synaptic connections to many other neurons, so there may be 100 trillion synapses in a human brain!
 
The fact that our neurons and synapses are so abundant as to be uncountable is probably why it is widely believed that "we only use 10% of our brains." This curious tidbit of folk wisdom is utter nonsense. There is no way to quantify the percentage of the brain that is "in use" at any specific time. And think about, if 90% of the human brain consists of unused "excess baggage," localised brain damage would not be a problem much of the time. In reality, damage in even tiny areas of the brain usually has severe, disruptive effects. The 10% myth appeals to people because it suggests that they have a huge reservoir of untapped potential. Hucksters selling self-improvement programs often disseminate the 10% myth because it makes their cliams and promises seem more plausible ("Unleash your potential!").