This is a difficult question to answer but we will give you all the relevant details and hopefully that will help you understand power bank charging a bit better.
These instructions primarily apply to the Carved Wood Power Bank.
The number of times you can charge your device from a power bank is dependent on 3 primary factors:
1 - What device you are charging.
The iPhone 4 has a 1,420mAh battery while the iPhone 7 Plus has a 2,915mAh battery. So the iPhone 4 can be charged twice as many times as the iPhone 7 Plus since it requires a lot less from the power bank for a 100% charge.
A good rule of thumb is to divide the mAh of your power bank by the mAh of your device battery. Now keep in mind this is never 100% accurate (see the next 2 reasons) but it will at least give you a rough idea. So a 6000mAh power bank would "theoretically" charge the iPhone 7 Plus from 0-100% 2 times (6000 / 2915 = 2.06). However you must keep reading to understand the other factors that are involved as this 2x number will almost never be accurate and will always be higher than what you will actually receive.
The same logic is true of any power bank size, 1000mAh/2000mAh/3000mAh/20000mAh/etc...
2 - What is the device doing while it is being charged.
If you are watching a movie on your device while you are plugged into the power bank, you may drain the power bank but it will appear that your device never charged. This is because you were taking the charge out of your device battery as quickly as the power bank could charge it.
If you leave your device on airplane mode and let it charge, you will get a lot more charges out of your power bank since the device is not taking much battery charge at all to run while the power bank is charging it up.
3 - The age and current charge of the power bank.
As lithium polymer batteries age, their capacity reduces. So a 6000mAh battery today may only have 5000-5500mAh capacity in 6 months. This is also dependent on the temperature of the area where you store your power bank. When a battery is constantly stored in high heat (think 80° Fahrenheit and more), it's capacity will go down a bit quicker than if it is stored in a cooler environment. Don't put it in the freezer though! Just store it inside, out of the sun and in a cooler area. That will help extend the life and capacity of your battery.
A battery may appear to be 100% charged but may only be 80 or 90% charged. Again, this will be due to the age and condition of the battery.
So as you can see this is a very difficult question to answer but hopefully the information we have given you here will be helpful!