When the word "number" is said and is followed by a figure, it is abbreviated except when it begins a sentence since it would look like hte word "no."
...It refers to Section No. 123.
...I am on page No. 15.
...Number 84 is not included here.
The plural of the abbreviation is "Nos."
...I have read Nos. 15 and 16.
Happy punctuating!
Margie