One hundred million is written with eight zeros. Since one million is written with six, adding the two more zeros for 100 makes a total of eight for 100 million.
One thousand has three zeros, 10,000 has four (one for the 10 in "ten thousand" and three for the thousand) and 100,000 has five (two for the hundred in "one hundred thousand" and three for the thousand). Ten million has seven zeros. One billion is actually 1,000 millions and has nine zeros. One trillion is a thousand billion and contains 12 zeros. The number called a Googol has 100 zeros, and another number called a Googolplex is written as a one with a Googol of zeros after it.