All your registered Parallels product subscriptions and licenses are stored in the Parallels cloud. After you have purchased a new Parallels Desktop license and started Parallels Desktop, you need to log in into your existing Parallels account or create a new one in order to activate Parallels Desktop.
After you have logged in to your account and activated Parallels Desktop, your subscriptions and licenses keys used for activation are automatically tied to the account.
If you have purchased a Parallels Desktop subscription with several licenses (you have received several license keys in the subscription confirmation email) all the license key are automatically registered as soon as you apply one of those keys and remain registered under the account.
If you have another Mac on which you need to activate Parallels Desktop, you will see the list of available activation keys as soon as you log in into your Parallels account from the Parallels Desktop interface on the new Mac.