The Prophet Muhammad (SAWA) has clearly said: My successors will be twelve. (Saheeh Bukhari, Hadeeth number 6682, Saheeh Muslim, Hadeeth number 3393, Tirmithi, Hadeeth number 2149 and hundreds of Sunni books as well as Shia books. The names of he twelve Imams were mentioned by the Prophet himself. We believe and follow the Prophet Muhammad (SAWA) by believing and following the Twelve Imams after him. The forst Imam is Ameerul Mo'mineen Ali (AS) and the last Imam is Imam Al-Mahdi (AS).
Any other sect who claim Imams apart from teh Twelve Imams need to proof their claim from authentic Hadeeths. We have not seen any authentic evidence about Imams apart from the Twelve Imams.
Ismailies, Bohras etc are Muslims like any other Muslim sect.