There is no theoritical limit to the number of administrators that can be logged on at the same time. However, we have found that any more than five or six concurrent, active users (of any type) tends to slow down the server.
CiscoWorks can be accessed from a remote workstation, that is the whole point of it being a web-based product. In fact, I have had Cisco engineers tell me that CiscoWorks actually runs better when you are logged on remotely.