Assuming you mean SMS site and not AD site then you can use this. If you want AD site you can change the view and column names. Appologize in advance for any typos.
select distinct model0 as 'Model', count(model0) as 'Model Count' from dbo.v_gs_computer_system gscs join dbo.v_ra_system_smsinstalledsites rass on gscs.resourceid = rass.resourceid where sms_installed_sites0 = @variable group by model0 order by 'Model Count'
Then create a prompt within the report. The name must be called variable or you can name it what you want but change the @variable above.
begin if (@__filterwildcard = '') select distinct sms_installed_sites0 from dbo.v_ra_system_SMSInstalledSites order by sms_installed_sites0 else select distinct sms_installed_sites0 from dbo.v_ra_system_SMSInstalledSites Where sms_installed_sites0 like @__filterwildcard order by sms_installed_sites0 end