How many black dwarfs are there in the universe?
None that we know of.
A black dwarf is a theoretical object which has not yet been observed. It is a white dwarf star which has cooled down to the point where it no longer emits light.
It takes a very long time for a white dwarf to cool sufficiently to become a black dwarf. It is thought that the time required is many times the current age of the universe. It is very unlikely that any black dwarfs exist yet.