How many themes have been released by LEGO?
In much the same way we had with the question on categorising bricks, I think this is something better handled by other sites out there:


BrickLink's list of Set Categories, or from their main catalogue page you can see a dropdown with themes listed.
Peeron's Theme Tree starts here, with the core LEGO System themes starting here.
BrickSet also has a list of themes, as well as the Theme Browser which lists the most popular on the page, and the complete set in a dropdown.