I am seeking to track some KPIs for my site, one of which is the number of activated, registered users there are on the site. Is there a way to get this number?
The Drupal way would be to use Views to create a report for yourself (I would avoid querying any tables in the db directly.)
If you don't have the Views module installed, download and install it from http://drupal.org/project/views.
Once installed, create a new View on the User object. You will be able specify which fields you want to display, sort order, and filters.
Depending on which version of Views you are using (might be Views 2 in Drupal 6) you may also need to use an additional module called Views Calc in order to get the count of records returned: https://drupal.org/project/views_calc