1. Find an Earth-sized planet
2. Must be away from the galactic center (to reduce radiation)
3. Must be away from the center of the galactic arms (radiation)
4. Probably best if it was not part of a multi-star system (orbital stability)
5. Must have water as a liquid (needed for life)
6. Must have much larger planets outside its orbit (to capture asteroids/comets/meteorites from coliding)
7. Must be old enough (tectonic stability, time to sweep out space debris)
8. Must have a large enough moon (to stabilize axial wobble), but not so large as to be considered a double planet (tectonics, weather)
and finally
9. Must be close enough to us (to be observable and remotely relevant).
Billions? I’ll guess that there’s two or three, if any at all.
We know there is at least one!