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.

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.