Nine. At the convention, they decided that trying to get all thirteen states to ratify the Constitution would be impossible, so they set the bar at nine.
The answer is 9 on Gradpoint
The approval of nine of the states was needed to ratify the Constitution.
On September 17, 1787, the Constitution was completed in Philadelphia. Benjamin Franklin then delivered a speech in which he spoke of unanimity, although only nine states were required to ratify the Constitution for it to enter into force.
After arduous struggles for ratification in several states, New Hampshire became the ninth state on June 2, 1788. Once the Confederation Congress received news of the ratification of New Hampshire, it set dates for the Constitution to enter in force, and the government created by the Constitution began operating on March 4, 1789.
Hope this helps!!