How many satellite stay in orbit?

Answer 1

Precise numbers of "spy satellites" are unavailable, and constantly changing. Dozens, certainly.

There are several different types of "spy satellites". There are photographic reconnaissance satellites, designed to take detailed photos of the surface. There are electronic surveillance satellites, designed to capture and record electronic signals such as radios.

Answer 2

That depends on the altitude, and the purpose of the satellite.

Low-orbit satellites are orbiting the Earth right above the atmosphere, where there is still a tiny trace of atmosphere up there. It doesn't take much; the super-thin air will still drag the satellite down slightly, into slightly thicker air, and the satellite will spiral down and burn up. If you have a satellite in low orbit, then you'll need to have thrusters or rocket engines to boost the satellite back up into its normal orbit, and do this fairly often. Even the International Space Station gets an occasional "bump" from the rockets that come to visit.

Higher-altitude satellites are safe enough, and satellites in GEO (geosynchronous Earth orbit) will probably never fall down on their own.

We intentionally de-orbit satellites if they are no longer functional, or no longer serve any purpose. Communications satellites can be discarded into interplanetary space with less fuel than is required for de-orbiting, but comsats are more likely to fail in place and be unreachable by radio control.

How many satellites? Thousands of intentional satellites, plus leftover space junk from the rockets that carried them up there. And when two satellites collide in orbit, the debris field of thousands of small satellite shards can spread out all over. NASA tracks nearly 20,000 satellites, fragments, and pieces of "space junk" larger than about 2 inches in diameter. (Smaller fragments don't show up well on ground-based radar.)