I am wondering that if i have two consecutive invocations of methods of a Stateless Bean then whether i am sure that the stateless instance obtained is the same or there might be different instances that do these invocations?
The heart of the question is that it doesn't matter if it is the same instance or different ones. As r035198x (jebus, what a name to type) already so succinctly stated, stateless beans should not keep any state. so it doesn't matter if it is a new instance for every call or the same instance. There is nothing you are going to miss when the container chooses to service you instance #2 in stead of instance #1 like in the last EJB method invoked.
I would focus my questioning on the parts of the EJB spec that ARE important to know, like transaction management and propagation.