Finally I found the solution myself.
In the activation config of the mdb itself there is a property called maxPoolDepth which defines the number of available mdb instances. But it is important that t ...
Check for this node :
<annotation expr="!class(@org.jboss.annotation.ejb.PoolClass)">
@org.jboss.annotation.ejb.PoolClass (value=org.jboss.ejb3.Stric ...