There is no hard limit for a number of devices or access points that can be managed on a Cloud Key, but there are some guidelines that are important to keep in mind. The most important factor to consider is not the number of APs or UniFi devices in general, but the number of concurrently connected clients.
For example, a Cloud Key would be a great option for a remote office with 2 APs and up to a dozen simultaneously connected clients. The Cloud Key makes it easy to manage the site remotely—and no one has to maintain a complex setup on-site. It could easily handle sites with many more clients or APs, but will always be best suited for deployments that can most benefit from the simplicity and ease of maintenance it offers. As the number of client devices goes up, the number of resources required of the controller increases. The Cloud Key is built for ease of install, portability, and convenience—but not for high-performance. For this reason, it's important to determine what type of UniFi controller is best to use for the needs of each UniFi deployment.
There are too many variables that will affect the number of connected clients or devices the Cloud Key can comfortably manage, so attempting to calculate an exact number is virtually impossible. However, to help visualization, this is what an aggregate of devices on a single Cloud Key might look like:
Cloud Key Generation 1:
Cloud Key Generation 2:
|10||USW (any model)|
As your network becomes more complex—with more sites, UniFi devices and most critically, connected client devices, it becomes a better option to use a dedicated controller with more resources than the Cloud Key.