Lisa, I don't have an answer for you on a maximum number of courses, but I have had users enrolled in many more that 300 courses (at least 1000). But it DOES make the interface very slow to load. Once the user is inside a course site, the slowness isn't as noticeable, but anywhere the list of courses has to be loaded (or searched) will be increasingly slow as the number of enrollments goes up. Basically what happens is every time the user goes to, for example, the Courses tab, a database query is made to find all of their courses.
Sorry not to have a solution for you, but at least confirms your user's experience!
--Lindy
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