What do you believe will be the makeup of the US Supreme Court by the time President Biden is out of office? If you believe that Biden will only serve one term, use 2024. If you think he’ll make it two, use 2028.
The size of SCOTUS will grow.
The size of SCOTUS will shrink.
The size of SCOTUS will be unchanged. (Currently at 9)
Any change would have to pass Congress. The scenarios that would make that possible are somewhat less plausible than Independence Day no matter how optimistic you might be about a Republican party split or collapse.
The number of sitting Justices on January 20, 2025 or January 20, 2029 may not be nine, for various reasons, but that’s independent of the actual size of the Court.
Since it seems remotely plausible that the size of the court would be expanded, but I haven’t seen any speculation about shrinking it, I assume that the OP intends to ask about “The number of seated justices”, but it would probably be worth clarifying here.
I’m going to vote for the second option as meaning “there will be fewer than 9 justices seated on the court” because that’s the only sensible interpretation of that option.
While obviously there’s no movement to reduce the Court below the current nine justices, I think there’s a good chance that Republicans could functionally remove a seat from the Court if they retake the Senate. If Clarence Thomas were to shuffle off this mortal coil the day after a new Republican majority Senate is sworn in in January of 2023, I have no doubt that Senate Republicans would happily deny Biden the opportunity to fill the seat for the next two years – or even six years if he is reelected and the Republican Senate majority holds.
Keeping a conservative majority on the Supreme Court is the highest Republican priority, bar none. It’s necessary to protect everything they’re trying to do now on voting rights and embedding themselves in power. There is no intransigence they will shy away from to assure their continued ownership of the Supreme Court.