The capacity figure on a power bank is not what you get out of it. A 10,000mAh unit delivers roughly 6,000 to 7,000mAh at the phone, because converting the voltage costs energy — that is physics rather than marketing.
Work backwards from your own phone. A typical handset holds about 4,000mAh, so a 10,000mAh unit is roughly one and a half full charges, and a 20,000mAh unit around three — enough for a weekend away from a socket.
Output matters as much as capacity. A portable charger rated at 18W or more will fast-charge a modern phone; a 5W one will only trickle it. Check the airline rules before flying with a power bank — most cap carry-on at 100Wh, which is roughly 27,000mAh.
