Create a column with type "column built from other columns". You probably want to set the "Search/Sort column by" box to "Number" so that you can use numeric sorting, and also searching such as #yourcolkey:>10.
Set the template to
For completeness, the above is a single-function mode template. The same template in template-program mode is
Code:
{tags:'count($, ',')'}
The same template in general program mode (runs faster than either of the above) is
Code:
program: count(field('tags'), ',')