At one extreme, a Business Group can be the only organization defined in the system. However this will severely limit your ability to manage and report on employee assignment and payroll information, and to control access to HRMS records. At the other extreme, you can define an organization for every grouping of employees, however small. But the costs of maintaining organizations at this level will probably outweigh the benefits.
When thinking about the internal organizations to set up for your enterprise in Oracle HRMS, consider what your current managerial groupings are. Then you can adjust up or down to determine the best level or organizational detail for your enterprise to maintain.