Stating outright the number of Jedi Knights in existence is always going to be a bad idea, because everyone has a different idea of them and their purpose in the galaxy. I imagine that the Jedi are a religious sect which isolates itself from society at large, much like Tibetan monks. They would have their own culture and family structure, and most would preach a philosophy of peace and would not have obvious mystical powers. It is only a very few who decide to serve the Republic and become knights, and these may not be the most powerful, only the most eager to serve the government. Since the Jedi are isolated from galactic society and their homeworlds are often peaceful (like Alderaan), it would be a simple task to hunt them down and destroy them. Since they are so isolated, regular people would probably only know of them, and not know or believe in their philosophy or powers of the Force. The Rebel Alliance would be the spiritual successor to the Jedi, which is why its leaders invoke the power of the Force.