user: Question:  how many candidates with result being Retired to run for U. S. Senate Republican hold
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Louisiana 1', 'Bob Livingston', 'Republican', 1977.0, 'Re-elected', 'Bob Livingston (R) Unopposed']
SQL:

assistant: SELECT COUNT("Candidates") FROM "Table" WHERE "Result" = 'Retired to run for U. S. Senate Republican hold';