I have a 12 Pro where is it that shows the percentage of battery left after use?

Open Control Center by swiping down from the top right corner of the phone.

Or if you want to constantly display the % on the home screen, add the Battery widget. See: Use widgets on your iPhone and iPod touch - Apple Support

From the top right hand corner of your iPhone, swipe down to open Control Center. There you can see the exact remaining battery percentage.