No. is the written short form of number. You put it before a figure when you are naming which one in a list: house no. 12, room no. 3, symphony no. 40. In speech you usually say “number”, not “no”. You will see it on doors, on forms, and in the titles of pieces of music, as in Mozart’s piano concerto no. 21. Though actually, American writing often uses the hash sign (#) for the same job, while British and older formal writing like No. It is not the word no that answers a yes/no question — it only looks like it. The little full stop is a clue that it is a short form.

第…號；編號（寫在數字前面；跟表示「不」的 no 不是同一個字）