For Apple devices, you can use your app on as many devices as you want, provided they share the same iTunes Store account or you are using Family Sharing. When you tap the Purchase or Restore button on another device with the same iTunes Store account, you will NOT be charged again.
For Android devices, you can use your app on as many devices as you want, provided you download them using the same Google Play account. If you have multiple Google Play accounts, select the account you originally downloaded the app with from the Play Store on the second device by tapping the Menu button, then Accounts and selecting the same account. Then, tap Menu and My Apps and you should then see it listed under "Not installed."