I realize that this reply is coming after your homework deadline, but I wanted to point out that there is no perfect way to gather this data. As an organization, Moodle HQ asks people to register their sites and collects a small amount of anonymised site usage data, but this is by no means a complete measure of all the Moodle sites in a given country. That said, it would be interesting to see a chart showing growth by country of registered sites over time. I will forward that suggestion to the web team.