For instance, let’s take a look at the next conversation:
A) How many questions does the test have?
B) A hundred.
And the difference between the conversation above and this one:
A) How many questions does the test have?
B) One hundred.
Are both conversations correct? if not, which one is correct?
They are both correct, and mean exactly the same thing.
I can certainly envisage situations where “one hundred” would be preferred; for example, in response to a question such as:
Q. Did Cruella de Ville steal one hundred and one Beagles, or two hundred and one?
A. One hundred. And it was Dalmatians you idiot, not Beagles.
Or, by contrast:
Q. How many press ups did you say you do every morning; was it a hundred, or a million?
A. Eh!? What do you think I am, a machine? It was a hundred. Obviously!
But even there, the “a” and “one” forms are pretty much interchangeable.
