I am confused about Ayatul Kursi where it ends? Some people say that this is Ayat ul kursi (single Ayat) so it ends at "Aliyul Azeem" Some people say it is Ayaat ul kursi (plural ayaat) so it ends at "Hum fiha khalidoon"
which one is right? Is there any narration from Aayimah Masomeen AS?

Yes we do have hadiths that it continues until verse 257. Shaykh al-Tusi narrates one such hadith in his bool Misbah al-Motahajjed page 758. So it’s better to read all three verses (255-257)