Hi,
that's hard to say as vanilla SAP UCES is interacting quite directly using SAP ECC. I personally would assume 400-500 concurrent users per SAP J2EE Instance on a state of the art server. However, as of the direct communication with the ERP/ISU backend you can't just keep on plugging in more and more instances.
Depending on your customer's need you have to consider to adjust some of the existing features to max out the amount of concurrent users you can handle.