Device Limit per user limits the number of devices a user can have. Ten is the maximum value that can be set.
When enabled, Silent Deactivation deactivates the first activated device after the device limit has been reached. It also allows new devices to be activated silently.
If Silent Deactivation is not enabled, then the system will not be allowed to activate the next device when disabled and the device limit is exceeded .
You can enable Group Management from the Hosted Setup page.