I don't think it can reach 50m unless it sold more than 5m on the PC.
by the end of this year I think it will be around 31.5 million.
Halo 3 and ODST and Wars can sell 2 million next year.
The total will be 33.5 million. if we assume that it sold ~2 million on the PC, the total will be 35.5 million. So, Halo Reach should sell ~15 million.
I don't think it will happen.









