Three to five: everybody does everything
One queue, no tiers, no specialisms. Adding structure here slows you down. The only discipline that matters is ownership: every ticket has a name on it.
Six to ten: introduce categories and a target
Categories for reporting, one first-response target, and a weekly queue review. Still one team, still no tiers. Appoint someone to own the queue — not to manage people, to manage the work.
Eleven to twenty: departments and a lead
Split into departments only where a genuinely different group answers. Introduce a team lead role with reporting and QA responsibility. Add escalation, because informal escalation stops scaling here.
Twenty to thirty: tiers and groups
First line and escalation, groups for shifts and specialisms, skills-based routing, and formal QA sampling. This is also the point at which an internal knowledge base stops being optional.
What should not change
Ownership on every ticket. A weekly look at overdue and stale work. And the principle that the customer belongs to the company, not to whoever answered first.