What works for 5 people breaks at 15, and what works for 15 breaks at 50. Identify which processes need to formalise at each stage, but resist adding bureaucracy faster than you add people.
Strong answers describe the transitions: from direct management to team leads, from informal processes to documented ones, from all-hands communication to cascaded updates, and from generalists to specialists. Best candidates discuss what to preserve from the small-team culture and what must change, and the common pitfalls at each scaling stage.
Senior leadership challenge. Candidates who have scaled teams know the specific pain points at each transition. Those who have not may underestimate the difficulty. Ask about specific breaking points they experienced.