How to make constraint when putting records into Access table? (1 Viewer)

I am developing a simple database for students to register courses using Microsoft Access 2010. I have 4 tables: Student, Register, Course, and Section (1 Course can have many Sections).

Then, there is a criteria that a student can only take up to 6 courses. Hence, may I know that where and how can I include this constraint? I believe that it should be in the Macro (Before Change) of the table Register, but I have no idea about it.

My relationship is: Student -> Register <- Section <- Course

Please help me with that! Thanks in advance

I wouldn't know how to do it with a macro.

With VBA I would have a form with a combo box on it in which I selected the student. I would also have a button on this form called "btnNewCourse". In the after update event of the combobox, I would run an SQL statement that returned the number of courses the student was taking. If this number was 6, then I would set the "Enabled" property of the combo "btnNewCourse" to false. It would now appear grey and unclickable preventing the addition of more courses.