This query will show the number of active workflows for each workflow type.
SELECT T.[Id], T.[Name], COUNT(*) AS [HowMany]
FROM [Workflow] W
INNER JOIN [WorkflowType] T ON T.[Id] = W.[WorkflowTypeId]
WHERE W.[CompletedDateTime] IS NULL
--AND W.[ActivatedDateTime] > DATEADD(d,-7,GETDATE())
GROUP BY T.[Id], T.[Name]
ORDER BY [HowMany] DESC
(The line's that commented out can be used to only show workflows that have been activated within the last 7 days)