Tomcat can serialize sessions out to disk so total number of active sessions is not a limit, assumng your sessions are Serializable. "Concurrent" requests are limited by the Thread and acceptCount settings - see your server.xml file. Here is one from Tomcat 5.5.20
The Thread here refer to the Thread Pool available to handle requests. As I recall, acceptCount is the number of requests that can be held pending the availability of a Thread from the pool. A google search for "tomcat acceptcount" turned up lots of confusion about this configuration.
subject: maximum concurrent session in apache tomcat