How many communicative civilizations do you think exist in our galaxy?
How many technologically advanced civilizations do you think are in the Milky Way? And how many might we be able to communicate with? The task of finding out seems daunting, but when you break the idea into multiple steps, it becomes a bit easier to comprehend.
The Drake Equation does just that. There are seven variables to consider. However, none of these values is completely understood, if at all. Once astronomers pinpoint values, we will have a much better understanding of the chances of life in the galaxy. Then we can answer the question, "Are we alone?" And we can follow it with an answer to Fermi's Paradox: "Where are they?"
To get a feel for the different factors involved with these huge questions, plug in values with your best estimates — and see how many civilizations you get.