Q. How do I abbreviate the word “number”?
A. Avoid num., which is normally reserved for “numeral,” and n., which in Chicago style usually means “note.” For casual prose, the number sign # is common. But even though “number sign” is its Unicode name, that symbol has a number (sorry) of other uses, including as a hashtag and in URLs that point to a specific part of a page.
In most contexts, the best choice is no.—a common form that abbreviates the Latin word numero. That’s what we prefer in source citations for journal articles, as in Critical Inquiry 49, no. 3, which refers to volume 49 (we omit the abbreviation “vol.” in this context), issue number 3, of that journal.