Do you mean the Drake equation? The relevant factor would be n_e "the average number of planets that can potentially support life per star that has planets"

https://en.wikipedia.org/wiki/Drake_equation

The thing with the Drake Equation is that it sort of plays with our lack of intuition around extreme probabilities. It includes some terms around the size of the universe and the number of planets that we (now) know are huge. And some others that we suspect aren't all that unlikely (like evolution of "life" in some form or other). But then it includes terms that, given a sample size of one, we really have no idea about--and just because 1 in a trillions odd seems a hugely conservative guess doesn't mean it's reasonable.