I don't know if there is a mathematical expression for it, but I asked this question- how often do you see one of anything in nature? Nothing, it seems is the only one of its kind, (with the sad exception of the last survivors of species going extinct).
I imagine that life in the universe exists the same way life on earth exists- anywhere it can. So we are not alone, though whether we will ever talk to each other is another question.

Terri