Calculate No Of Patients??

Hi guys

I have got a huge table which contains patients data.
I need to calculate the the no of patients for each clinical service.

Here is the sample data
Patient no Startdate EndDate ClinicalService cost
1qq 12/04/03 15/04/03 restHome $12
2qq 1/1/07 1/2/07 dementia $4234.00
3ww 30/01/04 12/01/06 hospital $1244
1qq 1/1/2004 12/1/2004 dementia $400


Assumption: If a person has been in two services at a different period of time, he will be treated as a different case each time in every different service.

I have got a table of dates called 'static_tab' which contains all the possible dates.
here is the script for the 'static_tab' table

USE [LOSDSS1]
GO
/****** Object: Table [dbo].[static_tab] Script Date: 11/13/2007 14:51:52 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[static_tab](
[cnt] [int] NULL,
[dates] [datetime] NULL,
[Quarter] [nvarchar](50) COLLATE Latin1_General_CI_AS NULL
) ON [PRIMARY]



I have been using a query for calculating total cost for each patient for each catagory . Its as follows

Select Year(x.dates) As [Year], Month (x.dates) As [Month],x.clinicalservice
Sum(dailyvol) As cost

From c.dates,f.clinicalService,
(f.cost / (DateDiff(dd, f.[Start date], f.[End date]) + 1)) As DailyVol
From tablePatients f

Inner Join static_tab c On c.dates Between f.[Start date]
And f.[End date]) as x
Group By Year(x.dates), Month (x.dates),x.clinicalservice

Order By Year(x.dates), Month (x.dates),x.clinicalservice




And the output table is as follows
clinical service Year Month cost
Dementia 2007 7 1016842.852
Dementia 2007 8 1005945.664
Dementia 2007 9 976899.9008
Hospital Care 2003 10 2714517.879
Hospital Care 2003 11 2724111.029

I want the similar output but insteasd of cost, I need the patient count.
I tried doing it but it gives me a conversion error

Please help guys

Thanks