user: Question: List the number of vacators when successors were formally installed on June 22, 1868.
Columns: ['State (class)', 'Vacator', 'Reason for change', 'Successor', 'Date of successors formal installation']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Delaware (1)', 'George R. Riddle (D)', 'Died March 29, 1867', 'James A. Bayard, Jr. (D)', 'April 5, 1867']
SQL:

assistant: SELECT COUNT("Vacator") FROM "Table" WHERE "Date of successors formal installation" = 'June 22, 1868';