what is the total number of publisher where cover date is may 1939
{'header': ['Character(s)', 'First Appearance', 'Cover Date', 'Publisher', 'Estimated Value'], 'page_title': 'First appearance', 'section_title': 'First appearances of popular heroes, villains and teams', 'caption': 'First appearances of popular heroes, villains and teams', 'rows': [['Superman', 'Action Comics #1', 'June 1938', 'DC Comics', '$2,100,000'], ['Batman', 'Detective Comics #27', 'May 1939', 'DC Comics', '$475,000'], ['Sandman (Wesley Dodds)', 'Adventure Comics #40', 'July 1939', 'DC Comics', '$60,000'], ['Namor the Sub-Mariner', 'Marvel Comics #1', 'October 1939', 'Timely Comics', '$400,000'], ['Jay Garrick/Flash I ; Hawkman', 'Flash Comics #1', 'January 1940', 'All-American Pubs.', '$95,000'], ['Captain Marvel', 'Whiz Comics #2', 'February 1940', 'Fawcett Comics', '$90,000'], ['Robin', 'Detective Comics #38', 'May 1940', 'DC Comics', '$75,000'], ['The Spectre', 'More Fun Comics #52', 'February 1940', 'DC Comics', '$72,000'], ['Lex Luthor', 'Action Comics #23', 'May 1940', 'DC Comics', '$8,025'], ['The Joker ; Catwoman', 'Batman #1', 'Spring 1940', 'DC Comics', '$160,500'], ['Green Lantern', 'All-American Comics #16', 'July 1940', 'All-American Pubs.', '$131,250'], ['Captain America', 'Captain America Comics #1', 'March 1941', 'Timely Comics', '$125,250'], ['Aquaman ; Green Arrow', 'More Fun Comics #73', 'November 1941', 'DC Comics', '$10,050'], ['Wonder Woman', 'All Star Comics #8', 'December 1941', 'All-American Pubs.', '$60,000'], ['Barry Allen/Flash II', 'Showcase #4', 'October 1956', 'DC Comics', '$48,000'], ['The Justice League of America', 'The Brave and the Bold #28', 'May 1960', 'DC Comics', '$8,127'], ['The Fantastic Four', 'The Fantastic Four #1', 'November 1961', 'Marvel Comics', '$28,896'], ['The Hulk', 'The Incredible Hulk #1', 'May 1962', 'Marvel Comics', '$21,672'], ['Dr. Doom', 'The Fantastic Four #5', 'June 1962', 'Marvel Comics', '$4,154'], ['Spider-Man', 'Amazing Fantasy #15', 'August 1962', 'Marvel Comics', '$45,150'], ['Iron Man', 'Tales of Suspense #39', 'March 1963', 'Marvel Comics', '$3,837'], ['Doctor Strange', 'Strange Tales #110', 'July 1963', 'Marvel Comics', '$3,500'], ['X-Men ; Magneto', 'X-Men #1', 'September 1963', 'Marvel Comics', '$13,545'], ['The Avengers', 'The Avengers #1', 'September 1963', 'Marvel Comics', '$5,148'], ['Daredevil', 'Daredevil #1', 'April 1964', 'Marvel Comics', '$3,160'], ['Teen Titans', 'The Brave and the Bold #54', 'July 1964', 'DC Comics', '$415'], ['The Punisher', 'The Amazing Spider-Man #129', 'February 1974', 'Marvel Comics', '$918'], ['Wolverine', 'The Incredible Hulk #180', 'October 1974', 'Marvel Comics', '$350']]}