First of all, it is about 10 km above the ground, which is just the flying altitude of a jet plane. This is higher than the summit of Chomolungma (Everest). Here, humans can no longer breathe. The air is that thin. However, when you look up, the sky is still blue. That means there is still air.
Now let's look a little higher, around 100 km above the ground. At this point, the sky becomes darker and darker. You can also see stars. Further up in the sky, 200 km above the ground, the space shuttle is flying. It is completely dark. There is almost no air. In other words, you have jumped out of the sky.
In this way, the distance between the sky and space is approximately 100 to 200 km. This is a very rough figure, but air does not just disappear at a certain point; it disappears little by little. Therefore, it is not possible to clearly draw a line that says "the sky is empty up to this point.

Even 200 to 300 km above the surface, only a small amount of air is present. Also, when measured rigorously, a very small amount of air seems to be present even above 1000 km.