How many Supreme Court justices are usually needed to decide a case?

What does this answer mean?

Usually five Supreme Court justices are needed to decide a case. The Court has nine justices, and five is a majority of nine.

The word "usually" matters. Not every case is decided by all nine justices, for example when a seat is empty after a justice retires or dies and the Senate hasn't yet confirmed a replacement. By law, at least six justices must take part for the Court to decide a case. That minimum is called a quorum.

The number of justices is set by Congress, not by the Constitution. It was six when the Court started in 1789, and it has been nine since 1869.

Because one vote can decide a close case, the choice of who fills an empty seat on the Court gets a lot of attention when the president nominates someone and the Senate votes.

At the interview, say "five." An easy way to remember it is that five is more than half of nine.