The Final Five escaped the nuclear holocaust on Earth several thousand years prior to the events of the series. They developed resurrection technology, and traveled (without faster-than-light drives, hence the amount of time it took) to the Twelve Colonies to convince the humans not to repeat their mistake and develop artificial life again. They were too late, as the humans were already at war with the Centurions, who had begun to create humanoid life (the Hybrids).
The Final Five gave the Centurions resurrection technology and developed eight humanoid models, the seven known models and "Number Seven," or "Daniel." The Number Seven line was apparently wiped out entirely by the Cavils (Number Ones).
So, apparently (as of this episode), there were actually 13 Cylon models, though only 12 now exist.