How many times could power bank 20000 mah charge the iPhone9?

The battery of iPhone9 is 4000mah, do you think power bank 20000 mah could charge iPhone9 for 4 times? let me show you the power bank 20000 mah first.

If some of the vendors told you that is power bank 20000 mah and it could charge the iPhone9 about 4 times, that the vendor is not professional.

As I know the power bank has an energy transformation ratio from the battery to the pcba to charge the device, normally this energy transformation ratio is around 66%

Let me show you an example, power bank 20000 mah is the real battery capacity inside, but when we are used it to charge our mobile phone device, after conversion rate 20000mah*66%=13200mah, which means it could charge an iPhone9 around 3.3times 

But power bank 20000 mah is really enough for charging iPhone9.