A valid license from an IGEL Workspace Edition (WE) Product Pack for each device.
Depending on the features you want to use, you might need IGEL Enterprise Management (EMP) licenses in addition. For further information, see IGEL Software License Overview.
If you have a smaller or medium number of devices and want to control exactly which device should get a license: See Manual License Deployment for IGEL OS.
If you have a medium or greater number of devices, and you are planning to add new devices/licenses frequently (licensing can be managed completely in the IGEL License Portal): See Setting up Automatic License Deployment (ALD).