If there are only 2 Sith...
In addition to the "Rule of Two" Sith, there are other groups of Sith stranded or sequestered on lost/hidden worlds that pop up from time to time. There are Dark Jedi (good guys gone bad) who are practically indistinguishable from Sith (and occasionally become true Sith). There are also ancient ghosts, for lack of a better term, of dead Sith Lords that sometimes possess or instruct Force users and thus recreate the Sith. So in a nutshell, there are not only 2 Sith in the galaxy, but that is not common knowledge in the Star Wars universe.
...Why aren't they easily defeated?
Classically, good guys follow the rules, even in a fight, while bad guys will cheat. Sith don't fight to uphold any ideals, they fight to win, and will do anything necessary to do so. Fighting an enemy like that without stooping to their level is difficult, and incidentally, is part of the reason many Jedi fall to the dark side.
Some fun titles to check out-
- Legacy of the Force series (9 books, starts with Betrayal by Aaron Allston)
- watch several Jedi (including a Solo and a Skywalker) drawn to the dark side.
- **Darth Bane series (3 books, starts with Path of Destruction by Drew Karpyshyn)
- when and why the Rule of 2 is born. Also, answers your question...**
- or just start with book 1 in any of the Star Wars series from this link [http://starwars.wikia.com/wiki/List_of_books]this link
I recommend starting with the "Tales from ________" short story collections listed about a third of the way down the page if you haven't read any Star Wars before. They are remarkable stories, well written, easy to digest, tie into the movies, and whet your appetite to dive into the novels.