We want you to form a good daily habit and not tire yourself out at the same time. Thus, you can subscribe to only one course at a time and take all the courses one by one.
If you don't already, maybe you should explain this on the site. Maybe allow the user to sign up for 2 or 3 courses at a time if they really want to though? You could still warn them before they try to bite off more than they can chew...
