If you looking for buying a power bank main attribute associated to a power bank is its capacity. This capacity is measured in milli Ampere Hour i.e. mAH. You can’t exactly calculate how many times a specific powerbank can charge your device, but the rule of thumb is more mAH rating mean more capacity.
Several reasons of buying a power bank is that it is portable and will charge your device. You can buy cheap ones with big capacity, but they usually will not charge your gadgets properly, or break down on you.
If you are looking to buy a portable power bank, we really recommend not going for the cheapest option. Only choose the best and mid range price power banks.