See the Drake Equation.
Some of the variables are now known to a reasonable degree of accuracy, so pick numbers you feel make sense for the remaining ones.
The original estimate was 1000 - 100,000,000 civilizations in the Milky Way galaxy alone.
We now know that R* and fp are higher than Drake's group originally estimated - though some of the other values (eg fi) are probably smaller.
As the values of the remaining terms are indeed unknown, you can justify both large and small values for each of them.
However, you cannot use zero for any of these terms because we already know that (semi) intelligent life has arisen at least once in this galaxy.
Given that the Universe is really rather big, it is extremely improbable that the remaining values are all so small that the final answer is 1 civilisation.