Missouri election laws permit candidates to get on a partisan primary ballot with a filing fee. This year, for U.S. Senate, there are 21 candidates on the Republican primary ballot, eleven on the Democratic ballot, and one each on the Libertarian and Constitution primary ballots. See this story.
The filing fee for statewide office is $500. Indigent candidates for statewide office can get on the ballot without the fee if they submit a petition of one-half of 1% of the last vote cast for the office.