Article V of the Constitution prescribes how an amendment can become a part of the Constitution. While there are two ways, only one has ever been used. All 27 Amendments have been ratified after two-thirds of the House and Senate approve of the proposal and send it to the states for a vote. Then, three-fourths of the states must affirm the proposed Amendment.
The other method of passing an amendment requires a Constitutional Convention to be called by two-thirds of the legislatures of the States. That Convention can propose as many amendments as it deems necessary. Those amendments must be approved by three-fourths of the states.