Technically it just requires 3/4 of the state legislatures (first 2/3 of them to trigger a constitutional convention, and then 3/4 to ratify its results).
Which - fun fact - is possible to do with states that together amount to less than 25% of the overall country population. In fact, given that it's really the legislature that needs to vote for ratification, and given FPTP being typical on state level as well + the usual gerrymandering etc that this enables, it's actually possible to amend the US Constitution arbitrarily with something like 10% of the voters (acting in concert to vote in the state legislatures that would then do the amending), if they have just the right geographic distribution.
Yup, which is why movements like https://conventionofstates.com/ can be quite frightening. As while it may seem reasonable at first glance, you needn't look very hard to find the sinister intent. Surely by now there has to be a name for the concept, similar to Godwin's Law, of right-wing movements bringing up George Soros. In addition, it mentions gay marriage, abortion, and the Affordable Care Act as examples of why a Convention of States is necessary. How lovely =/
Which - fun fact - is possible to do with states that together amount to less than 25% of the overall country population. In fact, given that it's really the legislature that needs to vote for ratification, and given FPTP being typical on state level as well + the usual gerrymandering etc that this enables, it's actually possible to amend the US Constitution arbitrarily with something like 10% of the voters (acting in concert to vote in the state legislatures that would then do the amending), if they have just the right geographic distribution.