You can take as many courses at the same time as you wish. In fact, the University is designed to not only act as a learning platform, but a reference resource. Even when you complete a topic, a lesson, or a course, all the information remains available for you to reference while you have a valid subscription.
You can always return to specific information and retrace your thoughts and the learning it provides.
The University does not unenroll you from a course when it is completed. It's all access, anytime, from anywhere, for any reason!