Shia traditional reports on the topic that Imams are twelve in number

H 105 - Muhammad bin Abdullah bin Ja’far has narrated from his father from Muhammad bin Isa from Muhammad bin Fadhl from Abu Hamza from Imam Muhammad Baqir عليه السلام that he said: “Surely Allah sent Muhammad صلى الله عليه وعلى آله وسلم to the Jinn and the humans and He has appointed twelve successors after him. Some have passed, while others are yet to come. The practice continues through each successor. The successors of Muhammad صلى الله عليه وعلى آله وسلم are on the practice of the successors of Isa عليه السلام and they are twelve in number. Amirul Momineen عليه السلام was on the practice of Masih عليه السلام.”