Server size depends on concurrent users (online at the same second), not total sign-ups. Total users = everyone who signed up. Concurrent = how many use the app in the same second — always a small slice, since most are offline. (Like a gym with 2,000 members but ~100 on the floor at peak — you equip for the 100.)
The rule: peak concurrent ≈ 5% of total registered users — a 1:20 ratio — the industry norm for self-paced apps (≈50% active monthly × ≈30% daily × ≈33% online at the evening peak). After launch this estimate is replaced by the measured number and the servers autoscale to it.
Change the selectors above — concurrent load, infrastructure sizing, and every option's monthly cost recompute live. All cloud figures are estimated minimums: list price × 18% GST × 5% safety buffer.