Yeah the context has a 1 to 1 relationship with web-apps (i.e. each web-app has 1 and only 1 context). However, as has rightly been pointed out a container, e.g. tomcat typically runs more than one web-app and so technically there can be more than one context in a container. BUT FOR THE EXAM YOU ONLY NEED TO KNOW THAT FOR EACH WEB-APP THERE IS ONLY ONE CONTEXT WHOSE SCOPE IS ACCESSIBLE TO ALL THE COMPONENTS IN THAT APP. You won't need to anything about containers with multiple apps.
Insofar as memory concerns I really don't know but I should imagine that as an object the context lives on the heap just like all other objects until they are garbage collectec, which in the case of the context object would not happen until tomcat is shutdown. The one thing I do know is this: THERE IS NO MENTION OF MEMORY MANAGEMENT FOR CONTEXT OBJECTS IN THE EXAM OBJECTIVES. So I would't worry about it too much.
Have you got the Head Firt Servlets &
JSP book? It's a great book and I think it could put your mind at rest on a number of issues.