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After she becomes fully-aware of her true self, Ellen reveals that the Sevens were destroyed by Number One (John Cavil) (TRS: "No Exit") decades before the second Cylon War. Cavil was angry at Ellen, their "mother," for playing favorites, and in a fit of jealousy he contaminated the amniotic fluid used to gestate the Seven copies and corrupted the genetic formula.
Ellen describes Cavil's wiping out the Sevens as more permanent than boxing, and Samuel Anders recalls that "Daniel died," confirming that no copies of Number Seven model survived. The remaining Cylon models other than the Number Ones have no memories of Number Seven. With this model no longer extant, Caprica-Six's statement, "there are twelve models," during the Fall of the Twelve Colonies was correct.
Cult of Daniel
After the episode "No Exit" aired, many fans on the official Sci-Fi Channel forum were convinced that Daniel was responsible for many of the mysteries in the show, was Kara Thrace's father, was responsible for the "head" visions of Baltar and Caprica Six, etc. This phenomenon, called the "Cult of Daniel" by Ronald D. Moore, was unintended, and Moore took the unusual step of unequivocally stating in the subsequent podcast for "Islanded in a Stream of Stars" that Daniel was not Starbuck's father and would not play a larger part in the series. Although he usually did not rule out any fan theories or explanations, he felt "the Daniel theory" had taken a life of its own and would lead many of its proponents to disappointment in the finale. He also apologized if people felt the writers were leading them to incorrect conclusions. The only purpose of Daniel was to explain the gap in the Cylon model numbers and serve as a "Cain and Abel" backstory with Daniel as Abel and Cavil as Cain.