Question about star wobble as it relates to planet discovery?

I watched an episode of The Universe on The History Channel and the episode discussed the discovery of extra-solar planets. Basically, how much a star wobbles can be an indicator of how much mass is orbiting it. This has led to the discoveries of many Jupiter-sized planets orbiting other stars. My question is, how do they know it is just one planet and not many smaller ones that equal the mass of the larger planet that is causing the wobble? I realize this is likely best explained with equations, but if possible please use a simpler explanation.