Forum:KB:Rule of two

are there really only two sith? why i'm asking is because after darth zannah there was darth cognus and after her darth millenial who started the rule of the strong again.

now things i heard and don't know if they're true. lord sidious is out of the line of darth millenial. and darth cognus had another apprentice after darth cognus. so there must be another two sith the whole time.

am i wrong?