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In the episode "Torn," Gaius Baltar questions Number Six as to why only seven humanoid models appeared on New Caprica and why he has seen only these same seven aboard the Basestar when she had told him previously that there were twelve models. Six replies that the Cylons do not talk about the other five models. Baltar, who has become worried that he may in fact be a Cylon himself, presses her on whether they would recognize one of the "final five." Their conversation ends abruptly, leaving the truth of the other models unknown.
In the episode "Hero," Number Three has a dream immediately after she dies, where she sees five glowing figures. Later, in the episode "The Passage," she tries to hold on to the memory of their faces and draws pictures of them soon after downloading, but the memories fade and the drawings are too surrealistic to be of practical use. It's clear that she does not consciously know what the final five look like. In the episode "Rapture," D'Anna and Baltar reach the Eye of Jupiter and D'Anna receives a vision of the final five. She seems to recognize one of them, but the vision is cut short before we see any detail of the figures. The shock kills her and upon awakening after downloading, she mentions her experience to Cavil. He confirms that the other seven Cylons do not know who the final five are and that the pursuit of that information is strictly forbidden by their programming. As punishment for this and for other acts, Cavil then terminates her and her entire line is "boxed", taking the secret of the final five with her.
In "Crossroads," four of the final five Cylons are revealed. Samuel T. Anders, Galen Tyrol, Tory Foster, and Saul Tigh begin hearing the same hallucinatory music that no one else can perceive (A version of "All Along the Watchtower" written by Bob Dylan). When the music draws them towards each other, a "switch goes off" in their minds and they realize that they are all Cylons. They resolve, nevertheless, to continue doing their duty as part of the fleet. Their "awakening" seems to affect other Cylons as in the episode "He That Believeth In Me," a Cylon Raider scans Anders then disengages the attack while on Galactica, Caprica Six tells Roslin that she can feel the final five and that they are near. It is later revealed in "Six of One" that the Twos, Sixes and Eights believe that the Raiders called off the attack because they sensed the final five, something the other models disagree with. Cavil further confirms that their original programmers programmed the seven Cylon models not to seek that information out.
Ronald D. Moore confirmed in an interview that the four are indeed full Cylons — however, "fundamentally different."
No mention of there only being one each of the final five models. I don't remember hearing that anywhere else.