The NAKIVO Virtual Appliance (VA) has two disks: the first one (20 GB) contains the Linux OS with NAKIVO Backup & Replication, and the second one (500 GB) is used as a backup repository.
If you deploy the Virtual Appliance disks using the Thin Provision option, then the disks will not reserve space on your datastore and will only consume space when actual data (such as your backups) is written to disks.