How many Drupal sites are there?

There can't be certainty as *exactly* how many Drupal sites there are.

Your best indication is at:

When you talk about Drupal sites, you talk about production sites, I presume - sites that are actually live, right? I currently have about 18 sites locally, and 6 live, although only 1 of them is listed on the URL above, because I have either not used the Drupal module, or I am not using Cron.

I don't know if there is any other way of determining the number of Drupal sites, but I guess something else you can look at (if those figures are available) is the number of times the Drupal core has been downloaded. While that will not tell you how many sites there are, it will certainly convince you of Drupal's popularity.

Regards,

Kobus