Where are these crazy figures coming from? The Pentagon, who has a stake in overestimating the size and capabilities of ISIS, tops out the high end of their estimate at 31,000 fighters. That's nowhere near the numbers being thrown out in this thread.
As for killing "hundreds of millions?" Dream big, I guess.
Let me correct myself a bit. When I say 100k I mean members not fighters. They may be willing to kill, but that does not mean they are capable. 30K trained, active fighters is probably on target, but if you include the hisba and the various other wings of dasch, it's a huge number.
They are never going to kill hundreds of millions of anything, but they are going to kill somebody.