The premium license you have purchased is linked to the account you used to make the purchase. So, you will not need to take a premium license for each of your sites.
You will just need to log in to the plugin with the email address you used to make the payment for the premium plugin.
You can download the premium plugin by following these steps:
1. Login to https://login.xecurify.com/ with the email address you used to purchase the plugin.
2. Navigate to the Manage License tab of the console.
3. You can find the premium plugin to download from here.
After downloading the plugin, navigate to your WordPress Admin Dashboard, deactivate and delete the free plugin and install the premium plugin downloaded. Please make sure to login with the premium email account.