True successor of Imam Jafar Sadiq a.s

There are couple of differences prevailing in Shia ithna ashari and Shia dawoodi bohra sects. The most important one is regarding imamat after imam Jafar Sadiq *a.s.* Ithna ashari sect believes that the true successor to imam Jafar Sadiq *a.s.* is imam Musa Kazim *a.s.* while Dawoodi Bohras believe it to be imam Ismail bin Jafar *a.s.* I need to understand who is the true successor, so that I can follow the right path. Even in Quran, it is indicated that the successors of Prophet Musa *a.s.* would be **12** and in several Ahle Sunnah books like Sahih Bukhari, Sahih Muslim, Musnad of ibn Hanbal, etc., it is focussed that the true successors of Prophet Muhammad *s.a.w.w* would be **12**, then it is clear that **12** imams are on *haq*, then why do Dawoodi Bohras follow *21* imams along with *Daee* after them? This can be considered as the second part of my question. *I would be grateful if someone may please enlighten me with references.*