Do I need to buy power bank based on my battery?

It is not necessary that power bank mAh should be higher than your phone's mAh. Suppose your phone's battery has 1500 mAh and you can buy 1200 mAh. This power bank (1200 mAh) can charge 50 to 75 percent of your battery (1500 mAh). The mAh is unit used to measure the battery power.

How many times I can charge my phone using power bank?

Suppose if your battery charge goes down to 0 percent and your are using power bank to charge your battery, if the can use power bank has more mAh it can able to deliver two to three of charge to your phone. It cannot be calculated exactly. But I found a formula in Facebook to calculate the how many times power bank can charge battery.

It is,

Real Capacity * Conversion Rate * Phone Battery Health / 1500

Where,

Conversion Rate is remaining percentage in your battery.

Phone battery health is draining rate in your phone.