EJB's consists of mainly the application logic at server side & are designed to perform operations irrespective of client application. In real-world enterprise applications, there can be various type of clients accessing them concurrently.
The EJB container manages every aspect of an enterprise bean at runtimes including remote access to the bean, security, persistence, transactions, concurrency, and access to and pooling of resources.
Container manages instances in the pool & is responsible for their addition/removal, increasing the size of pool real time.