You're looking for something like this?
Declare @StartTimedatetime,
@EndTimedatetime,
@Totalint
SELECT Client, datepart(hh, YourDate) HourNumber, SUM([NumRows]) VRowCount
INTO #1
FROM YourTable
WHERE YourDate BETWEEN @StartTime and @EndTime
GROUP BY Client, datepart(hh,YourDate)
SET @Total = (SELECT SUM(VRowCount)
FROM #1)
SELECT Client, HourNumber, VRowCount, @Total TotalRows
FROM #1