In BSG (2004), a massive Cylon fleet attacks the 12 colonies and wipes out, to our knowledge, all of humanity except for the fleet led by Galactica and Colonial One, and the Pegasus.
I suppose there are a couple questions here:
- was the Cylon fleet made up of their entire race?
- did they have a homeworld (in addition to the base at the black hole) where the rest were living normal lives?
It seems to me that if they had a homeworld that survived, the entire "the cycle is broken" plot point is actually not fulfilled. Humanity would evolve on a new planet (our Earth), but the Cylon skinjobs could still be around, 150,000 years further evolved.