How can I turn the battery percentage on??

On your iPhone 12 Pro Max, you will not be able to see the battery percentage when you are looking at the home screen. You will need to PULL DOWN from the TOP RIGHT CORNER of your screen and that will reveal the Control Center and you will see the battery percentage at the top right.

If you like to see the battery percentage without looking at the Control Center then you will need to work with the Battery Widget in iOS 14.x.x Install the large battery widget on your home screen and you will see the battery percentage at a glance.

Axel F.

I found that Apple put the percentage indicator in the Widget section... search for Batteries and swipe for up to 3 various displays and add the widget to the top of your screen like this (if you like)...