Nine states had to ratify the Constitution for it to take effect.
Article Seven stated that ratification by conventions in nine states would be sufficient to establish the Constitution among the states approving it. New Hampshire became the ninth state to ratify on June 21, 1788, triggering the Constitution’s legal effect.
The threshold was a compromise between requiring unanimous approval and allowing a small minority of states to create a national government. The remaining states could join later, and eventually all 13 original states ratified the document.
A frequent mix-up is answering 13, because there were 13 original states. Thirteen was the total number of states in the original union, but nine was the constitutional ratification threshold. Rhode Island, the last original state to ratify, did so in 1790.