How many witnesses are required to convict someone of treason

Article III of the Constitution  establishes a very high bar for convicting an American for treason. However, like other aspects of the Constitution, the exact meaning of this provision is being discussed by constitutional scholars. This reads the following:

Treason against the United States shall consist solely of waging war against them, or of adhering to their enemies, providing them with assistance and comfort. No person shall be convicted of Treason except on the testimony of two witnesses to the same overt act, or on the open court confession.

Explanation:

The explanation the prosecutions for treason are very rare is that the conviction requires two witnesses to the act of treason. In summary, by default, the definition of treason is quite narrow. This means that conviction for treason in American history is very difficult and therefore very rare.