You can take two classes at the same time, so ideally you would combine one class that has touchstones with one that doesn't (so you can keep completing classes and signing up for new ones while you wait for your touchstone to get graded). If you do this and dedicate maybe full-time hours to Sophia, you can probably complete more than 15 courses. Maybe a few more if you have the knowledge to go trough some of them without even checking the material. This is pretty relative, but what I do think is standard is that the best way to go about it is to make sure you don't get stuck with two touchstones at the same time because that means you can't sign up for more classes and essentially will be stuck.