2/3 of both Houses of Congress, which means, even if every Republican voted to remove Biden, he stays, unless 70ish Democrat Representatives, and 18 Democrat Senators, also voted to remove him.
That is a very large number, especially given that, if the vote is at all close, it will be political suicide for many of those Democrats to join with all(?) Republicans to remove a Democrat POTUS.
If they were to try the 25th Amendment, I expect that they would have to plead some intervening event that suddenly, and sadly, compels loyal Democrats to vote Joe out. If we get a spate of stories that it has just been discovered that Joe has had a series of strokes recently, then the 25th Amendment is in play.
So pleased that President Trump gave a pre-emptive warning to America's enemies not to try anything in the next few months.
More details available via links at:
Kamala Harris Ineligible Website (downloadable PDF available via link)
Twitter Site (twitter login required to view)