Mo. Rev. Stat. § 515.030: Parties may agree upon referees or court shall appoint.

In all cases of reference, the parties, except when an infant may be a party, may agree upon a suitable person or persons, not exceeding three; and, upon filing such agreement, the reference shall be ordered accordingly. If the parties do not agree, the court shall appoint one or more referees, not exceeding three, who shall be free from all exception.