Hello there,
Thank you for contacting Google.*
You can can use Google Play Music to listen your library on your computer and up to five smartphones. Up to ten devices can be used in total, including tablets, iPods, etc. While you can use multiple devices with the same account, you can only play music using one device at a time. Google Play Music All Access subscribers can listen to unlimited music on up to four mobile devices. Once you have connected 10 devices, you may add a new device only by*deauthorizing*an existing one from your account, and you may do this up to four times per year.
I've gone ahead and reset your authorized devices. You should be able to authorize up to 10 new devices and have 4 de-authorizations available to you. Please be aware that this is a one-time courtesy and for more information on managing multiple devices on Google Play Music visithttps://support.google.com/googleplay/answer/3139562?hl=en.*
Let me know if you have any further questions.*
Best regards,
Mike
Google Support Team