Abbreviation of "number (of)" in a table heading In a table, I would like to use abbreviate the word number. The column heading is namely: 


  Number of items


In what is the correct abbreviation for the word “numbers”? both "No" and "#" are suggested as abbreviations. 
The Chicago Manual of Style (16th ed.) in §10.43 also suggests "No". 
However, I am not sure these abbreviations are meant to be used with cardinal numbers. 

Question: What is the correct abbreviation of number in this context?
My suggestion would be to use #. The hash symbol is commonly used this way; in mathematics it's even used to denote the cardinality of a set, i.e. the number of items in a set.

Having said that, I think abbreviating it as No. is perfectly clear if your usage is along the lines of No. of x. If you think your audience will be more familiar with one of the two, go for that.