This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The container does not limit the number of threads that are allowed to execute concurrently in a servlet endpoint, like a servlet.
The EJB container, on the other hand, limit the number of threads that can execute in an EJB (with container managed concurrency) to one.
Thus you, as a developer, must take the above into consideration when developing the different kinds of endpoints. Or rather: You do not have to think about it when developing an EJB endpoint, but you must be careful with servlet endpoints.