In that case try:

Select Count(orderNo) as COrders, Month as theMonth into #TempCustOrders

From Orders

Where CustId = @CustId

Group by Month

Select t.COrders as CustOrders, Count(OrderNo) as TotalMonthOrders, Month

From Orders

inner Join #TempCustOrders t on t.theMonth = Month

Group by t.COrders, Month

Drop Table #TempCustOrders