Abbreviation of "number (of)" in a table heading
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.