what is the biggest number?
There is no biggest number; numbers go on forever!
However, if you are interested in seeing an almost unimaginably large number (called Graham's number) have a look at this Numberphile video: https://www.youtube.com/watch?v=XTeJ64KD5cg
You could say it's just infinite. Once, there was a contest on to type the largest number. It would obviously be 9999999999 and so on, because 9 has the greatest value.