Yes. Each plan has a limit on the maximum number of controllers permitted per account.
Homeowner & End User Plans
|Max Manageable Controllers
Note: As a contractor, the maximum manageable controllers include your own.
For example: If you have five controllers under your Contractor Starter account, you cannot add any more controllers. This means you will be required to upgrade to a Contractor Plan to increase your limit. In addition, if one of your customers has five controllers, this adds to the maximum number of manageable controllers.