how many devices can I use on Apple Music
How many devices can I use on the Apple Music subscription?
iMac, OS X Mountain Lion, G-Force external HDD
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My understanding is there isn't a limit as long as the device is using your Apple ID. However, you will find that you cannot play Apple music on multiple devices simultaneously, so you can't be playing on one device and a friend using another of your devices at the same time. For those purposes see:
Get an Apple Music Family subscription - Apple Support. - https://support.apple.com/HT205595 - including how to invite Family Sharing members