It has been anticipated that there may need to be a replacement justice during the late part of president Trump's term, as there was during Obama's. The Democratic president Obama nominated a replacement justice (Merrick Garland) and was knocked back by the Republican controlled Senate with the justification that it was an election year.
The Republican president Trump may nominate a replacement justice, and if he does so it is with a Republican controlled Senate. There is suspicion that the same justification that prevented appointment under Obama may not be applied. Presumably, this means that if they move fast enough, the Republicans could appoint another Supreme Court justice.
Given given the previous knockback of Obama's nominee, Democrats could reasonably claim a Republican late-presidential-term United States Supreme Court Justice appointment would be a hypocritical move, and try to prevent it or counteract it. The Republicans control the presidency and the Senate, the two participants in appointing a Supreme Court Justice, so the Democrats have no procedural method to prevent an appointment.
The number of Supreme Court justices is not constitutionally limited, merely by legislation. Altering this legislation is possible with the approval of both the houses of congress.
Polling is indicating that there is a reasonable chance that the Democrats will take control of the Senate in the coming US election, and a better chance of controlling the lower house. Even if the Democrats both houses and the presidency they will not be able to reverse an earlier Republican appointment.
Have Democratic representatives indicated that a late-presidential-term United States Supreme Court Justice appointment will be subsequently neutralized by stacking the court with, for example, another four justices? Or forty?