How many black holes are there in the Milky Way and why are there so many different scientific estimates?

Astronomers have got pretty good statistics about the number and size of stars in the Milky Way. Most end up as white dwarfs, while most of the heavier stars going supernova turn into neutron stars. Only very heavy stars collapse into a black hole when going supernova. We should know how many of them have existed since the birth of our galaxy and be able to calculate the number of black holes. We also know about galactic mergers in the history of the Milky Way.

There seems to be some controversy about what’s going on in the outskirts of our galaxy. I found this relatively new article, but a conclusive estimate for black holes seems difficult.

http://dsc.discovery.com/news/2009/05/11/black-holes-galaxy.html

Has anyone got some good information about this?

Milky Way May Be Teeming With Black Holes

May 11, 2009—Hundreds of relic black holes may be roaming the outskirts of the Milky Way galaxy trailing telltale streams of stars detectable from Earth, suggest astronomers in a new study. The black holes are crash victims, ejected from their original host galaxies when worlds collided, a process that Ryan O’Leary and Abraham Loeb, with the Harvard-Smithsonian Center for Astrophysics, suspect was instrumental in building our own galaxy and probably many others (...).

O’Leary and Loeb believe this was common practice in the formative years of the Milky Way, as small dwarf galaxies crashed into each other. The ejected black holes would not have enough velocity to escape the gravity of the newly combined mass and should still be wandering the outer regions of the galaxy today.

“An observational discovery of this relic population would constrain the formation history of the Milky Way and the dynamics of black hole mergers in the early universe,” the astronomers wrote. “A similar population should exist around other galaxies, and may potentially be detectable in M31 and M33,” they added. Each black hole is estimated to contain the mass of between 1,000 and 100,000 suns.