The right HR model depends on the work the business needs, not a fixed headcount rule. Some organisations need daily internal presence; others need experienced support for defined priorities.
Map the actual workload
Separate routine administration from recruitment, employee relations, policy, systems and strategic work. Estimate how often each need occurs.
Consider expertise and independence
Complex cases or major change may require specialist experience and an objective view. An internal role offers continuity and closer day-to-day relationships.