I don't remember the options on every generation of iPod Touch, but on a 6th generation model the only way to display the battery percentage is on the screen to the left of the first Home Screen. I believe it's known as the Spotlight Search Screen; it's the one with the Search bar at the top. Perhaps other generations are the same or similar*.
On the Spotlight Search Screen swipe up to reveal the lower part of the screen and you will see an Edit button. Tap that edit button and you will see the Add Widgets screen. The first part (the top list) shows widgets that are already on the Spotlight Search Screen and they have a - symbol next to them (to remove that widget from the Spotlight screen). Below that list is the MORE WIDGETS list; with + symbols next to them. Tapping the + sign adds that widget to your Spotlight screen.
Once you have done that, you can swipe right (that is, to the right) to reveal the left screen with the battery percentage on it.
* I have in the past noticed people suggesting variations to what I've told you, such as tap the Home button to get to the left screen and then tap it again to go left to the Spotlight Screen. That doesn't work on my 6th gen iPod Touch. Nor does any suggestion that one can show the percentage next to the battery icon on a Home screen. Perhaps that's how it works on other generations.
