Nine states had to ratify the United States Constitution for it to take effect.
Article Seven specifies that ratification by conventions in nine states would be sufficient to establish the Constitution among the states approving it. This threshold was a compromise between requiring unanimous approval and allowing a small minority of states to create the new system.
New Hampshire became the ninth state to ratify, on June 21, 1788. That vote placed the Constitution into effect under Article Seven, although major states including Virginia and New York had not yet ratified at that moment. North Carolina and Rhode Island joined later.
The nine-state rule is different from the final count of ratifying states. All thirteen original states eventually ratified the Constitution, but Rhode Island did not do so until May 29, 1790. The rule also differs from modern amendment procedures, which require approval by three-fourths of the states rather than nine states.