a question about JMS + MDB..

Hi guys. I have a standalone application that publishes messages to a topic. I'm using OpenJMS. I have another java application that connects to the JMS and retrieves the messages.
Now I'm trying to migrate the application to the J2EE environment, using Message Driven Beans (MDB). The question is:
1o.)if I'm publishing 1 message to a topic and the topic has many subscribers, how do I know how many subscribers there are? I need to get information about a subscriber (the mobile number) and send an SMS message to them, but I'm not quite sure how the OnMessage method of the MDB works..
any ideas..