Often times when I see the abbreviation for number, it has a period after it with a capital n so you can identify it from the word no.
Example: No. 15

Just my personal opinion, but I say that abbreviating makes things more confusing, and you might as well just say or write the whole word.

@crenshaw
Just because someone is black, you can't comment on how they can't talk.. but a black person could comment on how a white person talks? Stop double thinking. Apply your logic thoroughly to all situations. A person is a person, it doesn't matter what race.