No, you can always add more subscriptions and thermostats at any time to your SmartBuildings account. You can buy what you need, when you need it.
When you increase your subscriptions, you will pay a pro-rated amount for the remainder of the current term. For example, if you purchased a 1-year plan for 10 thermostats and add 5 more halfway through your term, you will only be charged for the new subscriptions for the remaining half of the term.
At renewal time, you will renew for the current number of thermostats in your account.