There are so many black holes in the Universe that it is impossible to count them. It's like asking how many grains of sand are on the beach. Fortunately, the
Universe is enormous and none of its known black holes are close enough to pose any danger to Earth.
Stellar-mass black holes form from the most massive stars when their lives end in supernova explosions. The Milky Way galaxy contains some 100 billion
stars. Roughly one out of every thousand stars that form is massive enough to become a black hole. Therefore, our galaxy must harbor some 100 million stellar-mass
black holes. Most of these are invisible to us, and only about a dozen have been identified. The nearest one is some 1,600 lightyears from Earth. In the region
of the Universe visible from Earth, there are perhaps 100 billion galaxies. Each one has about 100 million stellar-mass black holes. And somewhere out there, a
new stellar-mass black hole is born in a supernova every second.
Supermassive black holes are a million to a billion times more massive than our Sun and are found in the centers of galaxies. Most galaxies, and maybe all
of them, harbor such a black hole. So in our region of the Universe, there are some 100 billion supermassive black holes. The nearest one resides in the center
of our Milky Way galaxy, 28 thousand lightyears away. The most distant we know of lives in a quasar galaxy billions of lightyears away.