Presidential Name Frequency

6 James  James Madison, James Monroe, James K Polk, James Buchanan, James Garfield, James Earl Carter
5 John  John Adams, John Quincy Adams, John Tyler, John Calvin Coolidge, John F Kennedy
4 William William Henry Harrison, William McKinley, William Howard Taft, William Jefferson Clinton
3 George George Washington, George H W Bush, George W Bush
2 Andrew Andrew Jackson, Andrew Johnson
2 Franklin Franklin Pierce, Franklin Delano Roosevelt
2 Thomas Thomas Jefferson, Thomas Woodrow Wilson
1 All Others   Martin Van Buren, Zachary Taylor, Millard Filmore, Abraham Lincoln, Ulysses S Grant, Rutherford B Hayes, Chester A Arthur, Stephen Grover Cleveland, Benjamin Harrison, Theodore Roosevelt, Warren G Harding, Herbert Hoover, Harry S Truman, Dwight D Eisenhower, Lyndon B Johnson, Richard M Nixon, Gerald Ford, Ronald Reagan, Barack Obama

There are a few funny cases: John Quincy Adams, Stephen Grover Cleveland, Thomas Woodrow Wilson and John Calvin Coolidge tended to go by their middle names. Grant and Ford both changed their name (from Hiram Ulysses Grant and Leslie Lynch King, respectively). 

Even though it's the most common presidential name, there were no presidents named James between the death of James Garfield on 19 Sept 1881 and the inauguration of Jimmy Carter on 20 Jan 1977: 95 years, over half the lifetime of the republic at the time.  The second most popular name, John, went even longer: 4 Mar 1845 to 20 Jan 1961: 116 years, and George went from 4 Mar 1797 to 20 Jan 1989: 192 years.

addenda 14 Jan 2017: found out that Coolidge and Cleveland also went by their middle names so edited the article.  Surprisingly many presidents do not have a middle name given on wikipedia.  Here's the list of those that do:
J Quincy Adams, William Henry Harrison, James Knox Polk, Hiram Ulysses Grant, Rutherford Birchard Hayes, James Abram Garfield, Chester Alan Arthur, Stephen Grover Cleveland, William Howard Taft, Thomas Woodrow Wilson, Warren Gamaliel Harding, John Calvin Coolidge, Herbert Clark Hoover, Franklin Delano Roosevelt, Harry S Truman (no middle name, initial only), Dwight David Eisenhower, John Fitzgerald Kennedy, Lyndon Bains Johnson, Richard Milhous Nixon, Gerald Rudolph Ford, James Earl Carter, Ronald Wilson Reagan, George Herbert Walker Bush, William Jefferson Clinton, George Walker Bush, Barrack Hussein Obama.  27 of 43.