According to my carefully compiled calculations, a huge fucking lot of people are currently playing Halo 4.
Seriously though, I wouldn't trust these in-game numbers. They never include those who play campaign, custom games, forge or Spartan Ops. Only those playing matchmaking are tracked, so a lot more people are actually playing than it seems.
If you have the time though, I think you can find the answer at HaloWaypoint.com :) They track all kinds of shit.