Nine state ratifications were required for the Constitution to take effect in the founding era remembered by America250.
Article Seven specified that the Constitution would become effective among the ratifying states when conventions in nine of the thirteen states approved it. This threshold was a compromise: it allowed the new framework to proceed without requiring unanimous consent from every state.
New Hampshire supplied the decisive ninth ratification on June 21, 1788. The Constitution therefore took effect before all thirteen states had ratified it. North Carolina and Rhode Island joined later, with Rhode Island completing the original group on May 29, 1790.
This requirement is different from the process for amending the Constitution today. Article Five governs amendments and requires approval by larger supermajorities. Article Seven was a one-time rule for establishing the Constitution as the governing framework of the United States.