technically you are supposed to learn all of the speeches in case they specify one. last year they asked us for 3 speeches and every year before that they had only asked for two, so a lot of people got screwed who prepared their essays with only 2 speeches.
I was lucky, as our trial required us to do three speeches and that's what i had been practising. so my adivce is try to learn them all, but if u can't try to have a general idea of all of them and know 3 quotes from each so u could at least waffle in the exam.