Hello @Sali.Mahmud ,
If I understand your question correctly,
You should be using the same password, consistently, with the same DSS ID, by obtaining and supplying a valid DSS authentication token. You can find the authentication requirements described in REST API Tutorial 1: Connecting to the DSS server.
I think you are interested in issuing concurrent requests via DSS REST API. If this is the case, please refer to DataScope Select Best Practices & Fair Usage Policy for concurrent requests limits per template type, as well as concurrent processing guidelines and best practices (as an application designer, you may wish to submit requests up to the submission limit per template type, or only up to the actively processed limit).
Hope this information helps, otherwise, please explain more of your requirement?