[jessriedel]

There can be a lot. Given the immense volume above Earth, satellite collisions are only a worry because of the speeds that satellites travel and the current difficulty with coordinating them. As the technology for satellite coordination improves (tracking, low-propellent maneuvering, etc), the number of satellites that can fit will keep rising for a long time.

[londons_explore]

Geostationary orbit is a good example.

There, every satellite is following the exact same orbit, but different positions around it. There are 500+ satellites on it.

Now imagine how many possible orbits there are. If we could locate satellites accurately enough, we could give every 1 meter shell of altitude to another 500 satellites, giving us 2500000000 between earth and geostationary orbit.

[jessriedel]

The fact that there are only O(10^3) satellites in geostationary orbit is (I believe) mostly a limitation of ground antenna pointing accuracy than collision risk. There are many, many miles between each satellite along the arc.