limitations for session

Duration. Of course, that's only a limitiation if you want it to be.

Concurrency (as in how many can be run at the same time) - this is imposed by the web server software you're using, then by the amount of memory and hard drive space is available, and finally by the amount of bandwidth is available through your network/internet connection.

On the client side, it depends on the browser is being used and to a lesser extend, the user's hardware.