You are automatically enrolled for the courses in the 1st semester of your 1st year only. From the 2nd semester (period 3 and 4) onwards, it is your own responsibility to enrol for all courses (required courses, optional course profile ones and courses that are part of your approved optional minor). This means you will need to enrol for your 2nd semester courses yourself.
As of the academic year 2018-2019 you can enrol for a maximum of three courses (22.5 ECTS) per period. It is not necessary anymore to ask permission to follow a 3rd course. You can only enrol via Osiris Student and during the designated enrolment periods.