You can have one website connected to one Zyro subscription.
In other words, one plan purchase means that you will be able to upgrade one of the websites that you have built.
If you need to upgrade multiple websites, you will need to purchase multiple subscription plans.
You are able to buy as many different subscription plans as you need, and they can be of different durations too.
You can keep track of all of your subscriptions under the Subscription tab in your Dashboard.