Witnesses, Generally
Pennsylvania Wills are not required to have any subscribing witnesses.  As long as the testator possesses the required testamentary capacity, or sound mind, at the time of execution and has the ability to write his or her signature to the document, no one even needs to be aware that the testator is creating a will.

Although witnesses are not required, there are advantages to their use that must be considered.  Also, two subscribing witnesses are required for any will which is executed by another person on behalf of the testator. Any will which has not been personally signed by the testator and does not have two subscribing witnesses is invalid and cannot be accepted for probate

See: Witnesses, Advantages; Wills Without Witnesses; Self-Proving Affidavit; Witnesses, Eligibility