When using Tomcat 6.0, how many users can use the application at the same time? Is it possible to use more than 500 users can use the application same time?
It's not about how many users can access the system simultaneously, but how many request-response cycles it can process without break down. I suspect 500 users is big number, try Load Balancing.
There's no substitute for testing your code on production hardware against a reasonable user base.
Have a look at our Enterprise Performance FAQ for some starting points.