Mo. Rev. Stat. § 70.020

Whenever eight percent of the voters of each of any two or more contiguous counties, not exceeding ten, shall sign a petition, and shall file the same with their respective county commissions requesting the submission of the question of permitting said counties to perform a common function or service or employ a common officer or employee, it shall be the duty of said county commissions to submit the question to the voters of their respective counties at the next municipal election. The total vote for governor at the last general election before the filing of the petition whereat a governor was elected shall be used to determine the number of voters necessary to sign the petition.