What is the highest number?

There is no highest number. Higher numbers can always be formed by adding on to a previous number, a process that can go on infinitely. Infinity itself is not a number at all but an abstract concept used for describing something that is endless and limitless.

In 1938, mathematician Edward Kasner used the term "googol" to refer to a number equal to 10^100, or 1 followed by 100 zeros. He also used the term "googolplex" to refer to a number equal to 10^10^100, or one followed by a googol's worth of zeros. A googolplex is so large, even writing out all the zeros would require a time period much longer than the age of the universe.