NowClosed: the questions closed now. ClosedOnce: questions closed at least once. This query does not see the now deleted questions.
Q&A about the site for researchers and developers who explore the principles of a system through analysis of its structure, function, and operation
SELECT COUNT(ClosedDate) AS NowClosed, COUNT(*) AS ClosedOnce
WHERE Id IN (
SELECT DISTINCT PostId
WHERE PostHistoryTypeId = 10