Right now there are about ~7.6 billion people alive.
In total there have been approx. ~108 billion people.
Assuming ghosts exist, and when a ghost dies it becomes a ghost-ghost,
we can assume this 7% fraction exists for each remaining group of souls.
Thus, here's a population count for each level:
Seems very lonely for these last few ghosts...
A spooky halloween lesson by @rtaylz