I've had applications that had 2-3 users and required massive amounts of RAM. It's not the number of users that count, it's how the apps are written.
Tomcat doesn't have an internal virtual memory system that will go into page thrashing if you overcommit RAM. Instead the webapp will throw an OutOfMemoryException. That's your cue that you need more RAM, not a performance slowdown. Conversely, if you use the monitoring tools wisely, you may discover that your current Tomcat setup provides far more RAM than the apps need and you can thus reduce the Tomcat JVM's memory settings with relative safety.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.