There are 12 zeros after the one in one trillion. One trillion is equal to one thousand billion, which is the same as one thousand thousand million.
Since there are six zeros in one million and three in one thousand, a thousand thousand million has three plus three plus six, or 12, zeros. Similarly, since one billion equals one thousand million, it has three plus six, or nine, zeros. Quadrillion is one thousand trillion. It then has three plus 12, or 15, zeros. The Googol is written with a one followed by 100 zeros, and the Googolplex is written with a one followed by a Googol of zeros.