Вы можете попробовать это:
def m2m_save_slam_question(sender, instance, action, *args, **kwargs):
if instance.question.through.objects.all().count() > 2:
raise Exception(f'{self.slam.usercode} allredy 2 Question Is added')
m2m_changed.connect(m2m_save_slam_question, sender=SlamQuestion.question.through)