You can add new licenses at any time. If you need users to only receive, manage, and report the status of field assignments, purchase ArcGIS Workforce as a premium app add-on to an Editor user type. If you want users to have more robust capabilities, like using multiple field data collection tools from their mobile devices, the Mobile Worker user type is a single purchase that includes ArcGIS Collector, ArcGIS Survey123, ArcGIS QuickCapture, and ArcGIS Workforce.