More than the tool, what matters is the discipline of all functions (i.e., departments) agreeing on the business processes such as annual budgeting, quarterly/monthly forecasting, feedback loops to account for recent trends and key program-wise/location-wise assumptions (eg. attrition, shrinkage, training yield, ramp-up/learning curve etc.,).
Then, you build up a bottom-up forecast with those key assumptions that translates to revenue and expenses to build a P&L forecast.
A spreadsheet offers the most flexibility. Again, more than the tool, it is the process and the discipline (includes agreeing on standards, lead-times etc.,).
This culture and practices also varies from organization to organization and in practice, in large organizations, you have the legacy of acquisitions that cause a diverse set of practices. While a common tool might help these organizations streamline and standardize the headcount planning process, the challenges are not technical but human and cultural.