How to get different metrics for same table with one query?
Another way of writing the query (without using the  vw_EzProviderUser provider table because it is unused in the example  would be:

;WITH CTE AS
(
SELECT COUNT(DWB.CaseId) as CountingCaseId, DWB.CaseId,DWB.statusId
FROM
dbo.DashboardWorkbench DWB
GROUP BY DWB.CaseId,DWB.statusId
),
CTE2 AS
(
SELECT  COUNT(DISTINCT DWB.CaseId) as CountingCaseIdDistinct, 
        DWB.statusId
FROM dbo.DashboardWorkbench DWB
LEFT JOIN dbo.DashboardCaseStatusHistory DCSH on DWB.CaseId = DCSH.CaseId
GROUP BY DCSH.AssignedByUser,DWB.statusId
)
SELECT  SUM(CASE WHEN CTE.statusId = 9 THEN CountingCaseId ELSE 0 END) as metric1,
        SUM(CASE WHEN CTE.statusId IN (10, 1088) THEN CountingCaseId ELSE 0 END) as metric2,
        SUM(CASE WHEN CTE.statusId = 11 THEN CountingCaseId ELSE 0 END) as metric3,
        SUM(CASE WHEN CTE.statusId = 20 THEN CountingCaseId ELSE 0 END) as metric4,
        (SELECT MAX(CountingCaseIdDistinct) FROM CTE2
        WHERE CTE2.statusId = 12) as metric5,
        (SELECT MAX(CountingCaseIdDistinct) FROM CTE2
        WHERE CTE2.statusId = 10) as metric6
FROM CTE;


Performance

As of performance this could help depending on your data due to the early grouping on DWB.CaseId,DWB.statusId. 

YMMV 

There should be better solutions as to make it more performant such as columnstore indexes / better rewrites / indexing / ....

Temp tables

I would advise storing the result of CTE2 (or your subquery) in a temporary table if you are calling it multiple times. 

This way, you are only reading from the resultset, not evaluating the same query each time.

(Metric5 and Metric6 in the example).



Testing

This was tested on SQL Server 2017

When adding some indexes to cover all options:

CREATE INDEX IX_statusId_CaseId
ON dbo.DashboardWorkbench(statusId,CaseId);
CREATE INDEX IX_CaseId_statusId
ON dbo.DashboardWorkbench(CaseId,statusId);

CREATE INDEX IX_AssignedByUser_CaseId
ON dbo.DashboardCaseStatusHistory(AssignedByUser,CaseId);
CREATE INDEX IX_CaseId_AssignedByUser
ON dbo.DashboardCaseStatusHistory(CaseId,AssignedByUser);


The only real 'benefit' compared to your plan is not having the compute scalar operator
or the multiple SUM(CASE WHEN StatusId = 9 THEN 1 ELSE 0), statusid = 10, ... in your query plan on all data. 

In my rewrite, it is only doing that on the grouped count of DWB.CaseId &amp; DWB.statusId.

Part of the plan of your query on my test YMMV



The compute scalar from the part above (second from the right)



Wheras there is earlier grouping in my plan + count(DWB.CaseId)



This part of the plan represents the T-SQL statements in CTE (1)

SELECT COUNT(DWB.CaseId) as CountingCaseId, DWB.CaseId,DWB.statusId
FROM
dbo.DashboardWorkbench DWB
GROUP BY DWB.CaseId,DWB.statusId


the computes are done afterwards, on the result of the above query:



The rest of the plan is virtually the same.

Extra info

There will definetely be better solutions, but somebody might be able to use the DB&lt;>Fiddle below and test for something better.

DB&lt;>Fiddle with test data, execution plans etc.

Temp tables can help if CTE2 or your subquery is evaluated multiple times, indexes matter. And as mentioned in the comments there are other options to explore, but we would need more information.