On earlier versions of iOS, you had to turn on the battery percentage option to see this information. Not on the iPhone 12! These days, the battery percentage option is on by default—you just have to know where to find it.
If you are upgrading from an iPhone without Face ID, you'll see that because of the camera notch at the top, there's no longer enough horizontal room to display the battery icon and the percentage simultaneously.
Swipe down from the top right corner of the iPhone 12 screen to open Control Center.