Mo. Rev. Stat. § 545.330

The defendant shall be entitled to process for witnesses to be issued and directed to the sheriff of the county in which such witnesses may be; but all the witnesses in the same county shall be included in one subpoena, and no subsequent subpoena shall be issued for any witness, unless the court in which the cause is pending or the judge or associate circuit judge, shall for good cause shown, order a subpoena for another witness; or if, in absence of the judge, the defendant shall file with the clerk his affidavit that other witnesses ordered by him are material and positively necessary in his behalf, to a full and complete adjudication of the case, the clerk shall issue subpoenas for such witness.