Question

How many black holes are in our galaxy?

Answer 1

No one knows

Explanation:

Black holes have so much gravity that light cannot escape. As such they are not visible. We can see them if a bright object passes behind one in relation to the Earth (called gravitational lensing).

https://en.wikipedia.org/wiki/Gravitational_lens

The other thing that can be visible is the accretion disc of material around a black hole (things in orbit around it but not close enough to fall into it). This disc is called a quasar, but many black holes do not have them.

http://www.astronoo.com/en/quasar.html

So since we can only see black holes by the affect they have on things around them, if there is nothing nearby we will not see them. As such any guess to the number of black holes in the galaxy is merely a guess.

Since only 0.1% of stars have the mass to become a black hole, then one could argue that there is one black hole for every 1,000 stars in the milky way. This would be an estimated 250 million black holes.