What type of application? Web app? Standalone client accessing EJBs? Standlone client sending messages or web service requests? The maximum number of simultaneous requests for each of these is handled in different ways and in different configuration files. And none of those are related to the maximum number of simultaneous database connections, which is handled in yet another config file.
For web apps, max connections is handled by the adding an Executor entry to the server.xml file (located in server/xxx/deploy/jboss-web.sar). For details: http://tomcat.apache.org/tomcat-6.0-doc/config/executor.html
For database connections, specify max-pool-size in the *-ds.xml file.
By the way, I hope you are using JBoss AS 5.x, because the configs are different in earlier versions.
Yeah, I forgot it, I've been talkin' about a web app, thanks for your reply. Certainly, max-pool-size is under control. The first advice could be more viable, I'm gonna do it right now but before that, I gotta say what I'm using JBoss AS 4.2. I'll find out server.xml file and post about the results.
my application falls down when 4 users or more login and use it.
It looks more like a bug in your application? Or does your app is so resource consuming?