If congress and the president decide they want to increase the SC to 99 justices and add and additional 90 of their choosing then that's when the US Constitution officially dies but it can be done according to the law.
The attack on the legitimacy of the courts will fail this time around, just as it always has. The justices rule mostly with integrity and they’re mostly respected and protected.
What’s the difference between 9 of their choosing and 99 of their choosing?
It's easier to have public scrutiny on 9 justices than 99. There isn't enough time for the media or for people's memories to constantly remember to be angry at a few dozen people.
It's just a hyperbole for packing the court. Allowing one president to appoint >50% of the justices so that the court always rules in their favor instead of waiting for seats to be vacated and re-filled.
You have to kill some of them to put the others in place. If you add 90 slots you can just fill the new 90 slots with your guys. That’s the theory of his statement.