user: Question: How many times is the candidates Charles F. Mercer (F) 100%?
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Virginia 3', 'Henry St. George Tucker', 'Democratic-Republican', '1815', 'Retired Democratic-Republican hold', 'Jared Williams (DR) 64.6% John Smith (DR) 35.4%']
SQL:

assistant: SELECT COUNT("Incumbent") FROM "Table" WHERE "Candidates" = 'Charles F. Mercer (F) 100%';