Canonical Noddist lore holds that there are thirteen Antediluvians, one for each of the clans, and that each was Embraced by the Second Generation, meaning that they are all nominally siblings or cousins. Some of the clans believe in very different creation myths, however, particularly those that originate beyond Europe.
While most assume Caine had only three childer and thirteen grandchilder, this may not be the case. Several sources have offered different accounts, including:
1) The Erciyes Fragments states that there were more Cainites at the time of the Great Flood, but presumably few survived during the time which they had to feed on one another for blood. In particular, it cites the childer of Caine to be Enoch, Zillah, Jabal, Adah, Tubal, and Mehujael. It also suggests that the differences in generation came from a curse levied by Caine and did not appear until after his own childer were slain. (VTDA: Erciyes Fragments, pg. 81).
2) In a letter from Vlad Tepes to Mina Harker, Vlad stated that, in his research, he has come across various writings which state that the Third Generation numbered at 9, 27, 100, and 0. He wasn't sure which one was correct. (VTM: Vampire: The Masquerade Second Edition, pg. 13).
3) Ravnos lore claims that their founder, Zapathasura, was created by the gods and sired five childer: Black Mother, the Rakshasa, Chandraputra, Ravana, and Ramessu. Since this tale is incompatible with traditional Noddist lore, these childer may be members of the Third Generation, or they may all be faces of the same individual. (VTM: Clanbook: Ravnos Revised, pp. 18-19).