INTERROGATIVE PRONOUN NANI / NAN (何)
The general interrogative pronoun NANI (何) is used to ask the question WHAT.
何 is also pronounced NAN (なん) when the word that follows begins with d, t or n.
|Kore wa nan desu ka.|
Moreover, that modulation often occurs when asking about a quantity:
|How many (books)?|
|How many (people)?|
|Which day of the week?|
|Which day of the month?|