How many black holes do we know of?

Physicist Brian Cox explains "In terms of radio telescopes, we have a direct observation of two of them from the event horizon collaboration. One of the black holes in the galaxy called M87, which is about 55 million light years away, is so supermassive it's over 6 billion times the mass of our sun. We also have an image of this rather smaller supermassive black hole at the center of the Milky Way galaxy. We think that virtually every galaxy has a supermassive black hole at it's heart. Caveat a little bit – because a couple of papers have been released recently suggesting that there may be galaxies observed where there are no supermassive black holes. There are lots of galaxies in the universe, maybe there are exceptions, but it's fair to say that virtually every galaxy has a black hole at it's center."