Lost In Space showed many planets as having breathable oxygen in their atmospheres. How realistic is it for all these planets to have breathable atmospheres?
So what percentage of planets would be have breathable atmospheres with enough oxygen?
I note that the oldest fossils of lifeforms on Earth are at least three billion years old, but Earth did not have a breathable atmosphere until about 600 million years ago. The oxygen in Earth's atmosphere was actually produced by lifeforms through photosynthisis. So for at least 2 billion and 400 million years Earth was full of life forms which didn't need to breath oxygen and didn't need the present atmosphere of Earth. Remember that.
There actually is a scientific study about human habitable planets. Habitable Planets for Man, Stephen H. Dole, 1964.
There are many more recent discussions of planetary habitability. But in most cases I think they are discussing habitability for liquid water using lifeforms in general, not habitability for humans and other lifeforms which need oxygen rich atmospheres in particular. So a present day calculation that there are X billions of habitable planets in our galaxy will usually not say what proportion of those planets will have oxygen rich atmopsheres breathable for humans.
Thus Dole's calculations and estimations of the number of planets with breathable atmospheres will have to do, even though there has been a lot of astronomical progress in the almost 60 years since then.
On page 103 Dole concludes that there should be about 600 million human habitable planets in the Milky Way Galaxy.
On page 105 Dole says:
Throughout the Galaxy, the mean distance between a given star chosen at random and its closest stellar neighbor is about 4 light years; the mean distance between a star with a habitable planet and its closest neighbor with a habitable planet is about 24 light years.
So Dole calculated that there are a vast number of habitable planets with breathable oxygen atmospheres in the galaxy, but also that only a minority of stars have such planets and planets with oxygen rich atmossheres are separated by vast distances.