If JBoss runs out of http threads it shouldn't crash. I will just not server any more clients.
Th setup is done in "deploy/jbossweb.sar/server.xml" (JBoss 5).
Here is a quick introduction to the settings I found: http://tomcat-configure.blogspot.com/2009/01/tomcat-maxthreads-configuration.html
For more info look at the Tomcat docs.