Log in to your Parallels account at https://account.parallels.com/, open the Subscriptions and Licenses > My Subscriptions page, and click on the subscription you need a license for.
The click redirects you to the Subscription Details page, which displays the license keys that can be used for activating Parallels Desktop.
Keep in mind that registered license keys for some legacy Parallels Desktop versions might not be listed on the Subscriptions and Licenses page. You can find them on the Parallels Desktop > Legacy Licenses page instead. Later, those licenses will be automatically moved to the common Subscriptions and Licenses list.