Nine .
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At least 9 of the thirteen had to approve it.
3/4 of state legislatures :D
Article 7 of the Constitution was written to state that nine of the thirteen states had to ratify, or approve, the Constitution before it could go into effect.
9
Article VII of the US Constitution addresses this: "The ratification of the conventions of nine states, shall be sufficient for the establishment of this Constitution between the states so ratifying the same."