It would be amendment number twenty one, the repeal of prohibition.
The 21st amendment repealed prohibition and voided the 18th Amendment (the only amendment to repeal another amendment).
The 21st amendment, which is the repeal of prohibition, repealed the 18th amendment which was about prohibition.
The twenty-first amendment nullifies the eighteenth amendment.
The 21st which repealed the 18th (prohibition on the sale of alcohol)