> How many causes Kessler syndrome?

Space is huge. Try this trick: the number of satellites in orbit is about the same as the number of planes in the air at any time. (~12,000 [1].)

The volume of space from the ground to 50,000 feet is about 200x smaller than the volume from the Karman line to the top of LEO alone (~2,000 km).

Put another way, we approach the density of planes in the sky in LEO when there are milliions of satellites in that space alone. Picture what happens if every plane in the sky fell to the ground. Now understand that the same thing happening in LEO, while it occurs at higher energy, also occurs in less-occupied space and will eventually (mostly) burn up in the atmosphere.

Put another way, you could poof every Starlink simultaneously and while it would be tremendously annoying, most satellites orbiting lower would be able to get out of the way, those that couldn't wouldn't cause much more damage, the whole mess would be avoidable for most and entirely gone within a few years.

There are serious problems with space pollution. Catastrophic Kessler cascades that block humans from space, or knock out all of our satellites, aren't one of them.

[1] https://www.travelandleisure.com/airlines-airports/number-of...

You're ignoring the speed they're travelling at.

For a given period of time, a single satellite will travel through a vastly larger volume of space than a single plane.