Q. How do I enable Sub-Forums?
A. When creating a new forum (or editing an existing forum) you have the option to select a category; in this dropdown box you'll also see other existing forums. Upon selecting the parent forum you wish your new forum to belong to, and clicking submit, it will now be a forum within the parent forum.

Q. How many sub-forums can I create?
A. As many as you want, just keep in mind that it may be difficult for your users to navigate if you create your sub-forums too deep.