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.