How many witnesses testify in the US Supreme Court in a year?

They don't have witnesses give testimony in the Supreme Court. Instead, an attorney representing each side of the case conducts an oral argument presenting reasons they believe their perspective is correct. They may cite laws that support their side (usually contained in briefs) and answer numerous questions from the bench.

Each side is given 30 minutes to present its case, but the Court may grant extensions for complex situations or allot extra time for arguments from amici (parties not involved in the case who have an interest in its outcome), if the justices believe flexibility is warranted.