He canβt. His only power over SCOTUS is nominating Justices in the event of a vacancy.
This is wrong. He can pack the courts RIGHT NOW. The Democratic party still holds the Senate. There is no requirement for there to only be nine justices.
Edit: This does require the house changing the number of justices. So the above is incorrect.
He cannot. The Republicans have House majority.
The Constitution does not stipulate the number of Supreme Court Justices; the number is set instead by Congress. There have been as few as six, but since 1869 there have been nine Justices, including one Chief Justice.