If the power of attorney is being notarized, you do not need any witnesses. However, it is always a good idea to have 2 disinterested witnesses sign and be notarized as well in order to have additional proof of mental capacity and lack of duress or coercion.
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In addition to what Attorney Koel says, it is usually a good idea to have the witness statements eplain that the person is signing before them by making their mark.