Courses can only be created by any user with administrator role. There are no limitations on the number of courses that can exist on the system.
Course visibility can either be public or private, the former allows any user to enrol on the course (book) where the latter requires the user to be assigned. Private courses will not show up in the course listings unless a user has already been enrolled on that particular course.
You can set the status of a course to 'Preview', 'Active' or 'Cancelled'. There are other statuses but these are set by the system.
When creating or editing a course you will be able to select a theme which will change the background image on the details page.