How many mAh do I actually need in a power bank?

Reviewers generally point to 10,000mAh for one to two phone top-ups in a pocketable package, and 20,000mAh or more if you want multiple charges or to top up a tablet or laptop. Note that rated capacity is at the cell level: specialist communities repeatedly point out real-world delivered capacity is meaningfully lower due to voltage conversion losses, so plan for roughly 60-70% of the printed number reaching your device.