user: Question: How many DVD volumes was identified by Skippy Johnson?
Columns: ['Episode title', 'Villain(s)', 'Identity(ies)', 'Reason(s)', 'Original airdate', 'DVD volume']
Types: ['text', 'text', 'text', 'text', 'text', 'real']
Sample row: ['"Curse of the Collar"', 'The Ghost of Buster Mc mauler Mutt', 'Mr. Trixenstuff', "To steal Scooby's new collar for the jewels.", 'September 9, 1989', 4.0]
SQL:

assistant: SELECT "DVD volume" FROM "Table" WHERE "Identity(ies)" = 'Skippy Johnson';