First of all, starlink has been jammed in conflict zones.

Secondly, the number of satellites doesn’t really matter, the jamming happens at the end user level not at the satellite.

Finally, where are you getting that 20k number from? It’s not like there’s 20k starlink satellites in LOS to your station at all times. The earth will be obscuring many of those, they are below your feet on the other side of the planet.

Do they have atomic clocks on board?

I was under the impression that Starlink needs GPS for initial acquisition.

That's near orders of magnitude more than reality. The only number that matters is the number of satellites that are in communication line of site. A satellite on the other side of the world doesn't help you.

They've removed the in-built PNT service