Someone did genealogical research and claims that all the presidents of the United States descend from John Lackland. This is dubious but how many of the presidents are actually related?

Not at all dubious: the chances are that every person with any British heritage is descended from pretty much everyone in Britain if you go back far enough. The only thing that makes this story unusual is that this girl was able to find enough records to connect the dots and actually "prove" it. Here are a few posts on that particular story:

I've read that 43 out 44 presidents are linked to King John of England. Is this true?
According to the link in the description, Barrack Obama is technically related to King John of England, including a number of other presidents is there any truth to this?

Similar:

So my girlfriend met her birth father yesterday and apparently she's related to 4 presidents, Winston Churchill, and Joseph Smith among others dating back to the Mayflower, is there any validity to this family tree?