Schummer's threats of "packing the court" would require legislation signed by POTUS.

To change the judicial make up of the supreme court legislation would first have to pass the House with a simple majority.

Then once passed in the House, and assuming the filibuster is still in place the Senate would have to pass a cloture vote to bring the legislation to the floor of the Senate to vote up or down. The cloture vote requires 60 votes. After cloture the passage of the legislation on the Senate floor requires 50 +1 (I say 50 +1 because the +1 could be the VP or another senator).

Then POTUS would need to sign the legislation.


Now if Schummer is in control of the senate he would attempt to remove the filibuster however there is a big obstacle in his way. Cloture (or the filibuster itself...lol). He would not only need 60 votes to end cloture but he would need 67 senators present to end debate. That is a huge threshold for Schummer to overcome not counting the election threshold.

So in the final analysis Schummer's threat to pack the courts is for all practical purposes is a weak threat.