Most of the people in this forum have multiple Echo devices. Some of us have more than a dozen. The app is only required to set up a new Echo device. Once it's set up, you don't need the app any more unless you want to make changes. But on playing music, you can play different music on every Echo device you own all at the same time. When you play music, it doesn't actually play from the app (or your phone). You could play music from your phone if you use your Echo device as a Bluetooth speaker. Most people don't. Some pay-only music services (Spotify, Amazon Prime Music, etc.) may limit how many devices can be playing at the same time. I believe Spotify allows 2 on their basic plan, Amazon Prime Music only allows 1 (they want you to upgrade to Amazon Music Unlimited for a fee). There are plenty of free music services that allow you to play different music on as many Echo devices as you want.