There is no such "rule of thumb". I suspect that whatever you read was specific to that person's environment. Unless you do you own load testing you will not be able to determine what kind of load your system will be able to handle. And having two JBoss AS instances running on one server will not double the number of request threads that can be specified (or maybe it can, but it won't increase throughput). Youy have to remember that the more load you place on the system the longer it will take to process each individual request. It is a fine balancing act between the amount of load and the response time.