You can do something like -
SELECT A,B,C
,CASE WHEN ROW_NUMBER() OVER(ORDER BY A,B,C)=(SELECT COUNT(*) FROM dbo.YourTable)
THEN (SELECT COUNT(*) FROM dbo.YourTable)
ELSE NULL END AS RowCnt
FROM dbo.YourTable
If this doesn't help, please share the sql you have tried so far to get the o/p as shown in the question.
Hope, it helps :)