There’s a formula (Greenspan? Green-something-) that tries to estimate how many planets have life (or could sustain life, I don’t recall exactly). In essence it says that even though the probability of life is so small, the universe is so vast that it’s almost a certainty that it will happen a bunch of times.

Add to this we’re only looking for planets that could sustain life as we understand it.

And

Because the universe is so vast and old the chances of life happening somewhere else and us being able to observe it are still incredibly small.