How did Yoda know?
Yoda, at the end of TPM, after the death of Darth Maul and Qui-Gon, during Qui-Gon's cremation actually, said that "Two there are, no more, no less" obviously referring to the Sith. How would Yoda have known that, the Sith have been in hiding for 1000 years. He shouldn't have been able to know that there are only two Sith, he should have presumed that there were thousands or something and gotten really worried.