Apparently there are only 2 at any given time. A Sith and his apprentice. There are numerous Jedi Masters, thousands of Jedi Knights and apprentices and countless younglings. A veritable assembly line of Jedi. They even have a huge temple to hang out in. So why are Sith such a problem when they have to slink around in back alleys and dark corners? Is a Sith so much smarter and more powerful than all the Jedi combined?
Moving past the obvious "in the prequels, just one Sith was enough to destroy the Jedi and conquer the galaxy", I'll respond to this:
there are only 2 at any given time. A Sith and his apprentice. There are numerous Jedi Masters, thousands of Jedi Knights... why wouldn't one well motivated Jedi be sufficient to remove the Sith threat?
Think of the difference in numbers between, say, a real-life terrorist cell and the police and military of the country they're in.
Or the difference in difficulty between treating a cancer - controlling it, anticipating what it could do, balancing the need to control it wherever it might pop up with the side effects of doing so, then carefully, carefully trying to remove it without causing harm - with the ease of being a cancer that doesn't care what it destroys and simply looks to boost its power. As an oncologist I knew once said, treating cancer would be easy if we didn't have to keep the patient alive and healthy.
Committing terrible acts is easy when you don't care about the consequences. Preventing a ruthless individual committing a terrible act is extremely difficult when you do care.
It takes just one suitably-equipped amoral person to destroy something (be it a literal place or a state of order), and many to anticipate and prevent all the different approaches they could take.
As for how this can actually pan out, how individuals with powers and no scruples can have such an impact... Well, see the prequels, where just one Sith was enough to destroy the Jedi and conquer the galaxy :-)
- Turning one of your enemy into a monster, exploiting their weaknesses in any way that works, not caring how many innocents they take down while also taking down your other enemies, is much easier than trying to patch up all their weaknesses and keep them good
- Sparking wars is easier than stopping wars
- It's easy to make powerful allies if you're happy to help them do terrible things that don't harm your plan
- Fear is easier to create than remove, and easier to exploit than to cure
- Armies who obey orders without question are really handy
- Just one person can sow chaos, but it takes many to sustain order, and as a certain Sith-like character in another series likes to say, who also has disproportionate influence given that he's just one outsider: "Chaos is a ladder"