There are no fixed limits on any of those numbers.
However your web container may for example restrict the number of active HTTP sessions and your OS may for example restrict the number of open file handles. And at some point you will run out of memory and CPU.
Performance is far more related to what is being done and is far more complicated than simple numbers such as maximum numbers of users.
Alfresco support may be able to assist with practical benchmarking numbers for the enterprise releases.
