Know your current limits through load testing, not guessing. Then set capacity targets based on growth projections plus a safety margin for unexpected spikes.
Strong answers cover: analysing growth trends from historical data, load testing to find current limits, defining headroom targets (e.g. 30% above peak), auto-scaling strategies with appropriate thresholds, cost modelling for different capacity scenarios, and planning for organic growth vs sudden spikes (viral events, launches). Best candidates discuss the relationship between capacity planning and SLOs.
Practical SRE skill. Under-provisioning causes outages; over-provisioning wastes money. Candidates who have managed capacity during rapid growth demonstrate operational maturity. Ask: "How far ahead do you plan?"