In which cases am I required to take 6 hours of driving lessons?

If you have failed your practical exam twice in a row.
This means that you must have completed 6 hours of driving lessons before you take your practical exam for the 3rd, 5th, 7th, 9th, 11th, etc.
 

Apart from the above condition, you must also take 6 hours of driving lessons AFTER your provisional M36 or M18 driving license expires, so that you can apply for an M12 from the municipality.
 

Apart from the above condition, you must also take 6 hours of driving lessons AFTER your provisional M36 or M18 driving license expires and you want to take the exam based on an internship certificate. You can apply for this internship certificate from the municipality immediately after your M18 or M36 expires, but you can only take the exam after you have completed those 6 hours of driving lessons after the expiry of your M36 or M18.

If you applied for the internship certificate after your M12 expired, and you did not take a practical exam during the validity of your M12, you must bring a duplicate of the 6-hour certificate (with which you applied for your M12) to your new practical exam. If you did take an exam during your M12, this is not necessary.