The Halo series have amassed the staggering amount of 50 million sold units.
Starting back in 2001 with Halo: Combat Evolved, the series had sold 46 million copies until Halo 4 came out. It’s believed, or rather assumed, that Halo 4 has stood for 4 million of the total amount. The fourth installment was released on November 6th, 2012. During its first week, the game sold for $300 million.
So far Microsoft has released eight games since 2001. With the current sale figures, I don’t see a reason as to why Microsoft won’t release Halo 5 or Halo 6 in the near future. Following the recent release schedule, we can probably expect Halo 5 in 2013 and Halo 6 in 2014.