Buy a copy on one device, all other devices registered with that same ID can also install it without having to purchase an additional copy from that store. Keep in mind that the Apple App Store and Google Play are different stores and purchase at one store do not transfer to the other store.
For example, Dad has an iPhone registered to his user ID at the App Store. Mom has both an iPhone and an iPad, both are registered to her user ID at the App Store. Their son, Junior, has an Android phone. For everyone to have Prep & Pantry installed on each of their devices, three copies would need to be purchased. Dad would have to buy one copy for his iPhone. Mom would have to buy one copy also but she can install it on BOTH her iPhone and her iPad. Junior would have to purchase his own copy from the Google Play store under his own ID.