And this is correct. If the connection was only a ping pong call, then the answer would be thousands upon thousands.
The more complex your app it may be a bit slower. I may say hundreds upon hundreds?
Jetty the underlying technology is well proven and has good responses, so, then, my best advice would be: don't think about it too much.
"Make it work, then make it beautiful, then if you really, really have to, make it fast. 90 percent of the time, if you make it beautiful, it will already be fast. So really, just make it beautiful!" - joe Armstrong.
If you begin to feel that it is slowing because you have tens of thousands of calls, then just deploy a second server. Its that simple