Question

How many black dwarf stars are there estimated to be?

Answer 1

None.

Explanation:

A black dwarf is a white dwarf that has cooled to the point where it is emitting no significant tradition (heat or light). If we say that `5000`K is cool enough then it will take about `10^15` years for a white dwarf to cool enough.

The universe has only been around for about `1.37*10^10` years. So no white dwarf has had enough time to cool enough.