Its already happened a number of times. The original SCOTUS had only 6 members then they went to 5 then to 7, then to 9 then to 10 then back to 9.

IDK why the DNC is doing this.  Even if they increase the seats, Trump still gets to nominate right?

But what happens if there's a reduction?  Are the newest guys out?

IDK why the DNC is doing this.  Even if they increase the seats, Trump still gets to nominate right?

But what happens if there's a reduction?  Are the newest guys out?

Unless the necessary number of "extra" justices decides to retire or resign, the new max number will only be achieved through attrition.

There's currently no way to involuntarily remove a justice, even if they are ill and incapacitated and unable to perform their duties, short of impeachment.

IDK why the DNC is doing this.  Even if they increase the seats, Trump still gets to nominate right?

But what happens if there's a reduction?  Are the newest guys out?

Since a Justice can only be removed by Impeachment, if they cut the number
down they just don't replace those that retire or die till they get to the right number.
If the DNC tries to "pack" the court as FDR tried to do.  That may interesting.
Maybe we'll get to re-vist "Marbury V Madison and toss out this whole
rule by Judiciary fiat.   Federal courts were never given the kind of power they wield
today.
As Andrew Jackson said (paraphrased), "the Court has ruled, let the court enforce it".
Courts employ no enforcement people.  The enforcers all work for the president.
The president doesn't have to enforce Federal court rulings if he doesn't want to.

Andrew Jackson?! You rayciss! How dare you invoke that name!

When Marianne is president she has vowed to remove Jackson's picture from the White House. And rescind all the medals given to the soldiers who engaged in any violence with so-called "native americans".

No, seriously.

Williamson vows to rescind military medals, remove Andrew Jackson's Oval Office portrait in ‘atonement’ for Native American treatment

https://www.foxnews.com/politics/williamson-vows-to-withdraw-military-honors-remove-andrew-jackson-oval-office-portrait-as-part-of-atonement-for-native-american-treatment