Right now, after Justice Ruth Bader Ginsburg's death, we only have eight justices on the Supreme court. Should a difficult case, say a case about the 2020 presidential election, come before them, they might well deadlock in a tie 4 to 4 vote. In which case the lower court decision that was appealed to the Supremes will stand. Probably not good, the federal appeals courts only have jurisdiction over sections of the United States. If the Supremes cannot come to a decision then we will have sections of the country with one law and other sections with a different law. Not good. We ought to have the same federal law for the whole country.
Of course Trump will appoint the same sort of judge that he did for his previous two Supreme Court vacancies. That is a strict constructionist judge who will hew to the original intent of the Founders and not try to make new law from the bench. And to be fair, Trump appointees will either be Republicans, or at least be sympathetic to Republican viewpoints, and thus anathema to Democrats. No matter, I think the country will be better off having nine justices on the Supreme Court to rule on any 2020 election cases, which I judge likely to happen.