...in the Star Wars universe.
1) Every planet in the galaxy has roughly the same gravity, regardless of the size or composition of the planet, so everybody can move around comfortably.
2) Every species in the galaxy can breathe the same atmosphere (with a couple of exceptions), which is also the same on every planet.
3) Apparently, there are fewer than 900,000 droids in the entire galaxy, serving billions of sentient beings. How do we know? Star Wars Episode I makes it clear that "R2-D2" is an individual designation, not a model number. And in the entire canon, every droid is named using the same pattern. There is no instance of a droid with a number larger than 9 in its designation. Other episodes confirm that "R2" is the model type. So even allowing for letters or numbers in the 2nd and 4th place, that allows for fewer than 1,000 of R2-type astromech that seems to be everywhere, including on-board every fighter before they run out of numbers.