Hi Johny,
The server errors you observe can be caused either by some server-side problems (for example, when software, server hardware or some network hardware cannot process the increasing workload correspondingly) or by incorrect data sent to the server (for example, if multiple virtual users try to log in with the same credentials). In order to check whether the warnings you observe are caused by incorrect traffic, I recommend that you see the
Load Testing Troubleshooter help topic - it contains recommendations for HTTP 5xx codes.
The criteria for "how many user can connect and use their system concurrently" can vary depending on specific business needs, as mentioned by Alex above. They should be defined by the web site developers. They need to define a specific purpose for your test.
