How many Presidents have been related to each other?

This depends on how distant a relationship you accept. On a site devoted to descendants of royalty, I've read that about 1/2 of the US Presidents are descended from one of the Edwards of England. In the mid 1970s, I remember reading that Nixon, Ford, and Carter ( or at least 2 of them) had a common ancestor in England about 350-400 years ago.

From Charlemagne:
Garfield (possible disputed), Hayes, Pierce, Buchanan, Taft, Clinton (possibly), Madison, Harrison, Harrison, Cleveland, Ford, Bush, Bush, Washingtonm Jefferson, Quincy Adams, Taylor, Roosevelt, Roosevelt.
(My list give the earliest famous ancestor; the site lists under most recent royal ancestor.)
Above from

http://www.compapp.dcu.ie/~humphrys/FamTree/Royal/famous.descents.html#list

Try this source for more information. ( Scroll to near the bottom.)
http://www.compapp.dcu.ie/~humphrys/FamTree/sources.html#burkes.presidential

From another site, I found Coolidge and Hoover as royally descended, although from whom isn't indicated.

Coolidge was descended from Charlemagne, but not through the English royal line, apparently, as the others )above) were.

Doriangreyed has the more interesting answer, by far.

As for closer relationships, click here to find -

"George W. Bush (the 43rd president) is the son of George Bush (the 41st president).

John Quincy Adams (the 6th president) was the son of John Adams (the 2nd president).

Benjamin Harrison (the 23rd president) was the grandson of William Henry Harrison (the 9th president).

James Madison (the 4th president) and Zachary Taylor (the 12th president) were second cousins.

Franklin Delano Roosevelt (the 32nd president) was a fifth cousin of Theodore Roosevelt (the 26th president). Genealogists have determined that FDR was distantly related to a total of 11 U.S. presidents, 5 by blood and 6 by marriage: Theodore Roosevelt, John Adams, John Quincy Adams, Ulysses Grant, William Henry Harrison, Benjamin Harrison, James Madison, William Taft, Zachary Taylor, Martin Van Buren, and George Washington."