mathematics - Sorting a single number

What is the only number that, when written in English, gives a sequence of letters which is sorted alphabetically?

(The original phrasing of the question was meant to be a bit less straightforward to answer, as one had to come up with a way in which a single number can be 'sorted', but you didn't like it...)

Answer

I guess that the answer you are looking for is ...

... forty.

It is the only number whose letters are sorted alphabetically when written in English.