(a) The parties may by written stipulation, filed of record, stipulate that the jury shall consist of any number less than that fixed by statute.(b) The court may direct that one or two jurors in addition to the regular panel be called and impaneled to sit as alternate jurors. Alternate jurors in the order in which they are called shall replace jurors who become or are found to be unable or disqualified to perform their duties. Alternate jurors shall be drawn in the same manner, shall have the same qualifications, shall be subject to the same examination and challenges, shall take the same oath, and shall have the same functions, powers, facilities, and privileges as the principal jurors. An alternate juror who does not replace a principal juror may be discharged. However, if the court deems it advisable, it may direct that one or more of the alternate jurors be kept in the custody of one or more court officers, separate and apart from the regular jurors, until the jury has agreed upon a verdict. If one or two alternate jurors are called, each party is entitled to one peremptory challenge in addition to those otherwise allowed by law. The additional peremptory challenge may be used only against an alternate juror, and the other peremptory challenges allowed by law shall not be used against the alternates.