Learn Russian: Мне нужен ключ номер восемнадцать.

In Russian, when identifying a specific item in a sequence (like a room, bus, or key), you use the word номер followed by the regular cardinal number. You don't need to translate "eighteenth" here, just use the basic number for "eighteen".

Why is the sentence built with Мне нужен instead of a verb for "I need"?

Russian doesn't have a direct verb for "to need". Instead, it uses a structure that literally translates to "to me is necessary". Мне is the dative case of "I" (to me), and нужен means "necessary".

If a woman is speaking, does she need to change нужен to нужна?

No! The word нужен agrees with the thing that is needed, not the person who needs it. Because ключ (key) is a masculine noun, we use the masculine form нужен, regardless of who is speaking.

Will the number восемнадцать change its ending here because of the sentence structure?

No. When a number acts as a label right after the word номер, it stays frozen in its basic dictionary form. You don't have to worry about applying case endings to it.