A googleplex is grtting up there, that's 10 to the google power. A google is 10 to the hundredth power or 1 followed by 100 zeros. a googleplex is 1 followed by a google of zeros
There is no largest number! Why? Well, 1,000,000,000 (1 billion) can't be the largest number because 1 billion + 1 is bigger - but that is true for any number you pick. You can choose any big number and I can make a bigger one just by adding 1 to it.