On iPhone X and later, you can see the battery percentage in Control Center. Just swipe down from the top-right corner of your display.
On iPad, iPod touch (7th generation), and iPhone 8 and earlier, you can see the battery percentage in the status bar. Go to Settings > Battery and turn on Battery Percentage. When you use Low Power Mode on your iPhone 8 or earlier, the battery percentage will always show in the status bar.
Learn how to charge the battery on your iPhone, iPad, or iPod touch.