There was an FAA report released a few months ago that addressed this. You can probably google it or look on the FAA website (sorry, too lazy to go find it). It has projected staffing levels through 2022. I think it projected new hires peaking in 2013 and 2014 at around 1300-1400 per year - err something like that. Things will change significantly with NextGen because air traffic controllers will more or less become air traffic managers. Things will be more automated and less "hands-on" as more components of NextGen are implemented. The FAA reauthorization provided funding for things to progress further (finally) and I think the targeted completion is around 2022-25.
If you're asking this with the hopes of being a future ATC specialist, I HIGHLY recommend you attend a university to receive a CTI (e.g. ASU
, UND, ERU). There are other ways like getting a CTO or military service.
Hopefully this helps. Others probably have more to add that isn't a total crap post like mine