One hundred billion (100 000 000 000)
That's a lot of zeros, and the number is so big that it requires some sort of analogy to help place it in context. I like this one, from Daniel Levitin's book, "This is your Brain on Music":
Suppose each neuron was one dollar, and you stood on a street corner trying to give dollars away to people as they passed by, as fast as you could hand them out - let's say one dollar per second. If you did this twenty-four hours a day, 365 days a year, without stopping, and if you had started on the day that Jesus was born, you would by the present day only have gone through about two thirds of your money.
The American spelling of the scientific word for brain cells is "neuron", whereas the British (and most of the rest of the English speaking world) spell the word with an extra 'e': "neurone". Oh, and Levitin's book? Buy it. Now.