This depends on the capacity of the power bank you have and the device you intend to charge. For example, if the power bank you own has a capacity of 12000 mAh and the device you want to charge has a battery capacity of 2000 mAh, then your phone can be charged 6 times using the power bank (12000/2000).
However, there is an important point that needs mentioning here. In the aforementioned example, your phone can only be recharged 6 times in theory. In reality, it is unlikely to be able to charge your phone more than 5 times because there is some energy/charge that is lost during the whole charging process, which means full efficiency is not reached.