What do you call the highest number?

Answer 1

Sometimes "infinity" is used to denote "the highest number"

However, this usage is not correct.

The idea of infinite is that there is no bound to something; for instance there are infinite whole numbers. The problem with saying that "infinity" is the highest number is what happens if you add 1 to it? The new number you get is clearly 1 higher than the old number. This may seem paradoxical; the solution is that there is no highest number, there is always a bigger number.