I'm using the full version of Stage Traxxx 3 on an old MacBook air. I have another old MacBook air that I want to use as a backup device. I've loaded the app, but it is asking me to buy the full version to load more than 6 songs. Is this right? Do I need to buy the full version again for the second device? Thanks.

No, you don't need to buy it multiple times. You can use the app on any device running with your AppleID. Open Settings -> In App Purchases and tap on Restore Purchases to unlock full functionality.