How Many Days
What Day, Which Day
noun, expression
What day equals the fairly obvious translation "what day." But, it also means how many days.
なんにち
This is a jukugo word, which usually means on'yomi readings from the kanji, but in this case, we use the kun'yomi for 何. 日 uses the on'yomi.
アルバイト、何日休むんですか。
How many days will you take off from your part-time job?
何日がいい？
What day's good for you?
エレベーターから何日出られなかったんですか。
How many days were you stuck in the elevator?