Under the Articles of Confederation, all 13 states must agree in order for an amendment to pass.
The Articles of Confederation was America's first constitution after becoming independent from Great Britain. This constitution gave the federal government very little power, as Americans feared a strong central government like the one they experienced underneath British control.
With this in mind, the Articles of Confederation was developed so that states had a significant amount of power. Along with this, the Articles of Confederation made it difficult for the federal government to implement new laws. This is why it took all 13 states to add an amendment to the Articles of Confederation.