That depends when it would have passed. It was introduced for the first time in 1923, which would have made it the 20th amendment, just a few years behind the 19th, which granted women the right to vote.

However, I imagine you are referring to the 70's revitalization of the ERA, where it had a very real chance of passing. That would have made it the 27th amendment, the spot which is now held by the amendment limiting when and how Congressmen can increase their own salaries.