If you had asked "What types of listeners are there?" I would have pointed you to the servlet javadocs, where they are listened. But as it is I'm not making sense of the question. Are you asking how many listeners you can use in a web app? That's not part of the servlet spec; it's probably limited by available memory, but can't really be what you're asking.
Ping & DNS - updated with new look and Ping home screen widget