There are two "MaximumSize" entries in the jboss.xml files, and you will want to change the first one (defaulting to 15), in order to change the number of message listeners that are set up in the pool.
You still will not see ejbCreate() being invoked until the associated listener is actually assigned to service an invocation.
But don't do this. You will be happier if you override the number of instances on a per-MDB basis. Put the settings in your jboss.xml file, not standardjboss.xml. Here is what my jboss.xml file looks like where I set the pool size to 5.
<container-name>Standard Message Driven Bean</container-name>