Can I take power banks on airplanes?

Yes, in carry-on luggage only (never checked). Most airlines allow up to 100Wh (about 27000mAh at 3.7V). A 20000mAh bank at 3.7V is 74Wh—well within limits.

How many times can I recharge a power bank?

Quality power banks last 500+ charge cycles before capacity degrades. At daily use, that is over a year before you notice reduced capacity, and they remain usable for several years.

Best Power Banks to Keep Devices Charged All