The principle for discussion on this Constitution Monday comes from Article I.3.6, "No members of the judiciary or the executive branch of government shall be convicted of impeachment charges unless there is a concurrence by two-thirds of the members of the Senate in attendance." This principle gives the accused person the right to protection from partisan politics because it requires the attendance of a substantial majority of the Senate.
Because there are currently 100 members of the Senate, a quorum could be constituted with 51 Senators in attendance. Two-thirds of 51 would be 34 so a President or any other executive or judicial officer could be impeached by a relatively small number of Senators. In reality, impeachment proceedings are so rare that there are usually plenty of Senators in attendance to provide for fairness. If all 100 Senators were in attendance at the proceedings, 67 would need to vote for conviction.