There were only 12 types of skin-job Cyclons in the new Battlestar Galactica. Why this (low) limit? Was this ever (in-universe) established?
The explanation is that there were 8 models created by the original "Final 5" cylons up until the point where the Cavils rebelled against them, during which one of the models was destroyed/left mentally dead.
There may actually be more...
As the other answers explain the Final Five are the five remaining Cylons from Earth 1, which are the decedents of the 13th tribe of Kobol, who were basically advanced apotheosis-seeking humans who found ways to extend life and were exhiled by the other 12 tribes for doing so.
"The Centurions were already trying to make flesh bodies.They had created the hybrids, but nothing that lived on its own, so we made them a deal.You stop the war, and we'll help you. We developed the eight humanoid models, and we gave them resurrection." - Samuel T. Anders, No Exit
From the Battlestar Galactica episode, No Exit, and The Final Five comic book mini-series, we learn from Ellen and Anders that after a deal was made with The Guardian Centenarians from the 12 Colonies, upon the Final Five returning to Kobol, that they would help them advance by creating 8 new models. The first being in the image of Ellen's father, John Cavil. John however sabotages the #7 line also known as "Daniel", who never appears in the series.
However, Caprica, a prequel TV series to Battlestar Galactica, taking place at 50 years before the fall of the 12 Colonies, tells the tale of Zoe Greystone and how through her avatar, Zoe A, was able advance's Zoe Greysone's father, Daniel Greystone's technology by believing in a monotheistic belief system (The Solders of the One).
The series did not go beyond the first season, but ends with Zoe A getting a human-looking body so that she can exist outside of V-World and no longer look like a Centurion robot.
No one in the Battlestar Galactica era is aware that Skinjob Zoe existed because her father went to such pains to keep her secret.
The EPs of that series also released some tentative plans for season two, which included at least one of the Final Five, possibly Galen Tyrell, making contact with Zoe A on V-World and thus suggests that there may have been a lot more going on (and a lot earlier than Anders said) in order to achieve the results that ended the first Cylon War; that we may not actually know for certain if Zoe A (or potentially others) actually cease to exist...
Note - there is also whatever exactly was going on in Blood in Chrome with the reveal at the end that included Tricia Helfer voicing the humanoid cyborg....
I'd strongly suspect that it's a religous thing. Remember this the Cylons are beings that believe in a monotheistic God. Perhaps to them, 12 is one of their sacred numbers, as might be 5.