user: Question: what is the total number of publisher where cover date is May 1939
Columns: ['Character(s)', 'First Appearance', 'Cover Date', 'Publisher', 'Estimated Value']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Superman', 'Action Comics #1', 'June 1938', 'DC Comics', '$2,100,000']
SQL:

assistant: SELECT COUNT("Publisher") FROM "Table" WHERE "Cover Date" = 'May 1939';