How many witnesses are required to an act of treason to be convicted?

There must be two witnesses to the same overt act of treason to convict a person of treason, unless the person confesses to it in open court. See the US Constitution, Article III, Section 3, Clause 1, which states:

"Treason against the United States shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court."