What is the number of disks reserved for Data ONTAP OS on the NetApp filers?.
There are no Disks reserved for Data Ontap. Usually OS resides on Vol0(root volume) which resides on aggr0(root aggregate).
Mailbox is of two disks resides at first aggregate which is an alternate heartbeat path for the cluster software.
root volume/aggregate can be renamed.
OS itself is located on internal boot device (USB flash in current models). Settings are in vol0, which can be in any aggregate (minimal size is model dependent)
If Clustered Data ONTAP (cDOT) there are 3 disks reserved for mroot as the factory default. Root (mroot) has to be on a dedicated aggregate with cDOT, however for 7-Mode it is optional to put vol0 on a shared or dedicated aggregate.