When purchasing Video Background Pro, a license key is generated for you. Additionally, the email you used to purchase Video Background Pro becomes your license email for that generated license.
Each license key has an amount of activations associated with it. Currently, Video Background Pro sells 1 site, 5 site, and 25 site licenses. This amount of activations reflects how many WordPress sites you can have the plugin activated.
You can view all of your licenses at the License Keys page of your account.
If you have lost your license, you can recover it here. Remember to recover the license with the email address that you originally purchased the plugin with, as it is associated with your license key.