Quick answer: Launch week traffic is hard to predict. A capacity budget based on wishlist count, similar-game launches, and headroom multipliers gets within 2x of reality.
Most indie launches over-provision (waste money) or under-provision (lose players). The 2x estimate is usually right.
Start from wishlist
5-15% of wishlist plays on day one. 30% within week one. Scale infrastructure to peak D1 + week-one tail.
Compare to similar launches
Find 3-5 launches with similar wishlist size and genre. Their published concurrent player counts inform your estimate.
Add 2x headroom
Plan for 2x your best estimate. Surprises happen; running out of capacity on launch day is catastrophic.
Have a scale plan
Auto-scaling configured before launch. Manual scaling, knowing the unit cost. Both ready; auto-scaling preferred.
“Launch capacity is the most expensive thing you'll over- or under-provision. Spend the time on the estimate.”
Save the launch-week capacity post-mortem. The data informs every future launch; institutional memory is the cheapest form of accuracy.