There are 12 Cylon models.
This was not always in the case. In 1978, Battlestar Galactica was very different than its 2003 reimagining. Both shows have the same basic plot: the enemies of humanity, the Cylons, are able to wipe out most of humanity in a devastating attack, leaving a ragtag group of survivors aboard the titular ship.
In the original series and its follow up, Galactica 1980, the Cylons were created by a reptilian race that has since been completely annihilated. In the modern remake, it is revealed that humanity created the Cylons, some of which is chronicled in the spin-off prequel series Caprica. The original Cylon look was very distinctive, a large silver helmet with one haunting red eye in the middle. This look was preserved and updated for the original, but using CGI effects, the Cylons are much larger and terrifying than originally conceived. When the survivors aboard Galactica flee to start humanity anew, and the Cylons give chase.
Another change to the original concept of Cylons was making "humanoid" models. The first one we meet is Number Six, as played by the seductive and deadly Tricia Helfer. Over the course of the miniseries and four seasons, we meet the rest of the Cylon humanoid models. Noteworthy among them are Number Three, played by Lucy Lawless from Xena: Warrior Princess fame, and Dean Stockwell from Quantum Leap as the villainous Number One. Little is revealed regarding how these Cylons function; while created from technology, it's discovered early on that they are nearly indistinguishable from normal human beings. While they are sometimes shown to be stronger and faster than normal humans, they are also portrayed as having the same weaknesses, including emotional vulnerability and the ability to get sick.
While my all time favorite episode of the show is "33" from the first season, where the Cylons remain a mostly psychological enemy for Galactica's crew, my pick to discuss has to be the second season, episode "Downloaded." This episode departs from it's standard story structure; instead of following members of the Galactica crew, it follows two Cylons that were instrumental in their defeat. The first is a Number Six, dubbed "Caprica Six" by the other Cylons, who was instrumental in gaining access to restricted military information ensuring the Cylons' ultimate nuclear holocaust. The second is "Boomer" — a Number Eight model who we know better as Sharon Valerii. Both are having problems in this episode re-"adjusting" to Cylon life. We learn a great deal about the Cylons in this episode, including seeing how they "download" into a new body after death.
Want more science fiction TV? Check out the previous blog posts on Star Trek: TNG and The X-Files. Also, remember to stop by Mid-Manhattan Library to see all the awesome Sci-Fi Summer events taking place June through August.