Starlink's orbits are so low that everything deorbits automatically. The satellites need to actively work to stay up. That means no Kessler syndrome there.
How many you can fit depends on the available technology. It should eventually be a lot more than 70K just in those low orbits... and still leave plenty of space for rocket launches and returns to thread their way in between them.
> Starlink's orbits are so low that everything deorbits automatically.
It is enough if it goes one round around. They can make a cascading effect which can destroy tens of satellites at once, and few fragments are enough. And closer to earth you are, less space there is. They can't all orbit on exactly the same level. There is always one which is on slightly higher level.
Humans are bad at intuiting exponents. There is roughly 200x more volume in LEO than there is between the ground and cruising altitude. Plane changes, moreover, take a lot of energy--you aren't going to get enough energy out of a collision to pollute nearby orbits.
> going to get enough energy out of a collision to pollute nearby orbits.
There is no infinite space. The problem is exactly defining the number objects when that "small" amount of energy is actually enough to cause problems.
