Do you know there are a lot of narrations in Shia’s sources, which mentioned the number of twelve Imams?
Shaykh Ḥorr-e ʿĀmelī in the second volume of the valuable book “Ithbat Al-hudat” has gathered nine hundred and twenty seven narrations about the imamate1 of twelve Imams, which in most of them the number and name of Imams have been mentioned straightforward.

For example;
۹۵ narrations in “Kafee” of Koleynee
۵۳ narrations in “Oyoon” of Sadoogh
۲۲ narrations in “Ma’ani” of Sadoogh
۹۲ Hadith in “Kamal Al-Deen”
۲۲ Hadith in “Amalee” of Sadoogh
۱۸ narrations in “Al-Ghaybah” of Toosi
۱۱ narrations in “Misbah” of Toosi
From the article of Ayatullah Muhammad Mahdi Asefi