وعليكم السلام ورحمة الله وبركاته Those scholars who say that the Taraweeh prayer cannot exceed eleven Rakaa don't not allow the extra Rakaa that you have mentioned in your question . But I tell you my brother , the statement that the number of Rakaa is limited is clearly wrong according to nearly all the previous Ulema which the scholars of the four Madhaahib altogether. Also Imam Ibn Taymiyyah described this opinion as a mistake and a delusion.. and same with Imam Ibn al Qayyim and Imam Ibn Hazm . So there is nothing wrong with the increase that you mentioned in your question. Know that the confusion which you are in is one of the evidences for the weakness and incorrectness of that opinion.