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More people regret what they said than what they didn't.
Literal
Be-silent-was [thing-こと] [object-を] regretted person [more-than-より], talked [thing-こと] [object-を] regretted person [subject-が] is-many.
Aphoristic comparison built on the parallel frame X を後悔した人 vs Y を後悔した人, joined by より ('than') and concluded with 多い ('numerous'). The two relative clauses 黙っていたことを後悔した and しゃべったことを後悔した contrast people who regret their silence with people who regret their speech — literally 'people who regretted having stayed silent are fewer than people who regretted having spoken.' A common pattern for comparative wisdom statements.