Hi Marco,
The answer will differ depending on which OnCommand Insight module you're deploying.
For Assure, Perform, Plan - the licensing is done by raw TB (2^40 bytes) for the discovered arrays.
For Balance - the licensing is done by usable TB. You need only license the usable capacity from storage pools with monitored workloads (i.e. pools with monitored servers / VMs attached to them). For NetApp, this would translate into the formatted capacity the aggregate. There is also a controller-based license available for FAS / V x2xx controllers.
Hope that helps.
Regards,
Bryan