How many indefinite articles are there in English?
There are two indefinite articles in English.
A is used when the following word starts with a consonant.
An is used when the following word starts with a vowel.
When deciding whether a or an is used you analyze pronounciation.
That is why noun hour should be preceeded with an. Although in writing the word starts with a consonant "h", in speech this consonant is silent and the word starts with a vowel.