Hi Selvam,
the number range is assigned by business partner grouping, as this is one of single mandatory attributes of business partner. You can have more roles per one business partner and you can add them later to an existing business partner.
Check the documentation in IMG (SPRO->Cross application components->SAP Busines Partner->Business Partner->Basic Settings->Number ranges and groupings->Define groupings and Assign Number Ranges.
You have to make sure in your process to choose proper grouping when creatin employee or sold-to-party.
Regards,
Dawood.