A googolplex.
There isn't one because numbers are infinite. It gets to the point where we can no longer name them and they become just a string of digits, but they do go on forever.
Chat with our AI personalities
There is no largest number under infinity because what ever the highest number is, one can always be added to it, making the new number the highest number.
There is no highest number
There is no highest number, highest prime number, or highest composite number. Therefore, there can't be a second-highest, third-highest, etc., of any of these types of numbers, either.
The highest single number in the decimal system is 9. The highest single number would vary depending on what base you are using, for instance the highest single number in the binary system is 1. The is no 'highest' number. There is a concecept call infinity but that is not really a number but a mathematical tool. There is no such thing. Anything number that you can think of you can always add one more to it. Some people might say infinity is the highest single number, but infinity is a concept not a number. Look at some of the other answers on here related to infinity for more details.
There is an infinite number of possibilities to the highest number.