By default, you can only enroll up to 16 devices under one username. You can lower the number of devices threshold. If you attempt to enroll a device after reaching the threshold number, the device with the oldest "Last Seen" time is automatically force-removed.
Guide on how to lower the number of devices threshold.
Login to Zscaler Admin Portal.
Under Policy, click Zscaler Client Connector Portal.
it will pop-out another window.
under Administrator, select Client Connector Support > Device Cleanup tab.
under Force-Remove Oldest Device After User Enrolls, select the number you desire, between 8 to 16 or can be Never.
when selected desire value, you can login your username to multiple devices within that value. if you select Never, by default it will login up to 16 devices.
you can select also for Automatically Force Remove Inactive Devices After and Permanently Delete Removed Devices After.