In Enterprise Manager, I select the database of interest and then go to the Performance page. The second chart has a tab named Services. If you click on this, you can see the load broken down by service name.
You can get a count of the sessions by a query similar to the following:
select service_name,inst_id,count(*) as num_connections
group by service_name,inst_id
order by service_name,inst_id;