the number of alien civilizations in our galaxy right now equals...

the rate of star formation x
the fraction of starts which have planets x
the average number of planets per star that can support life x
the fraction that actually develops life x
the fraction which actually develops intelligent life x
the fraction of which are willing and able to communicate x
the expected lifespan of a civilization

these values are of course highly disputed and we can only speculate, but as we learn more we can get closer and closer.

i like the way carl sagan fills this equation in. he puts higher values in for the whole thing than anybody else but comes up with a smaller number, only because of time. he believes that a civilization's lifespan is the reason it is so unprobable that there are just teams of civilizations going on right now in our galaxy. it is very huge out there and it is very old, if civilizations only last even say average 10,000 years, that makes it damn near impossible that there are going to be all these civilizations living right now in the milky way.